Okay, so maybe not the *literal* devil, but pretty close. 

I bought a mare back back in August, her name is Kate’s Hot Chex and her previous owners have always referred to her as Katie/Katie girl. I have a bias about the name and saw an opportunity to have a horse named after my favorite home remedy for the common cold or just being cold and started calling her tottie, like a hot tottie. And she is definitely hot. Lady I got her from said she was getting a feed I don’t have access to (I drove three hours to pick her up) but gave me a couple different recommendations on similar feeds by national brands that I did have access to. So I started feeding her Purina Strategy, and all was well I think she actually got fatter after I brought her home and I didn’t think that was possible. The mare is HUGE. Anyway so I’d ridden her once before I bought her, jumped on her bareback with a rope halter and a lead rope as soon as I got her out of the trailer, rode her a couple more times around my arena, she did great. Til one day, she didn’t. This is where the literal devil thing comes in. So I got her all saddled up this particular day, she stood perfectly fine never acted annoyed or anything about any of her tack. Stood perfect at my mounting block, never flinched when I picked the reins up and put my foot in the stirrup. My butt barely touched the saddle and she decided I was going to die right then and there. I’m talking NFR-level bucking bronc. Threw me to the ground before I could even tell what was going on and proceeded to stomp on me not once but three times and bucked her way across the arena. Broke my reins and everything. Heifer is CRAAAAZY. So I put her on the lunge line the next day (I legitimately thought she had shattered my hip or at least dislocated it so I went to the hospitsl or I would’ve done it as soon as I could stand up lol) and she goes nuts. Like I’m talking jumping like a cutting horse in front of a cow when I’m just trying to get her started in one direction. Crazy. Then she’s running harder than I’d ask a horse running a barrel pattern to run, and I haven’t even put any pressure on her with the whip or anything. Just wants to RUN. mind you, at some point or another she was supposedly a trained pleasure horse. Not meant for running lol. So I got as much energy as I could out of her and she’s still acting loony. Called a friend in the next day cause I knew if she threw me again I would definitely not make it out without something broken, lunged her again with the same running as hard as possible results, and she tried to get her head down enough to throw my friend (who only held on because she was expecting the fallout, thank you.) after maybe 10-15 minutes I got on her and she was a perfect angel. Crazy horse. I’ve got her off the strategy and she seems a little less hotheaded, but she’s still moody as can be. I bought her a blanket, first night she let me put it on fine. Second time I went to put it on her she tried to kick me. :| I don’t understand how people deal with mares. 

Moral of the story... anyone want a horse under their Christmas tree? Lol

OK, First and foremost, thank goodness you are okay! I have seen some hot Quarter Horses in my time, but geez. I have wanted a horse, but not like that lol.

I would ask if you know if she has any back issues or nerve sensitivity since sometimes it can make a horse turn from 0 - 60 with riding, but you've already said you rode her perfect before this moment. So, clearly that is out lol. Secondly, what is the feed she suggested you don't have access to? Is there a brand closest to it? (Besides the Purina Strategy?).  It seems food and temper are kinda related since she got less looney switching from Purina.

What I can't get past though is how this mare was said to be a pleasure trail horse. Every pleasure trail horse I have ever seen are so laid back and obedient. The Chex line primarily does cutting and reining, like her attitude shows. I think someone tried to do trail on Tottie, but it didn't work out for the same reasons you see now. But I have never ridden, just ground worked so I am not an expert there.

With the lunge line, though, maybe you could try a double line? I knew a barrel mare who was super hot and they had to double her line because she couldn't be controlled (she'd run like your mare,Tottie, and buck or pull on the line). She got better on the double and calmed right down, but I don't know if that's the right way or not (their methods were, questionable, to put nicely). Though typically, a lowered head in a lunge line means they're submissive or relaxed. In Tottie's case, it sounds like she's dropping her head to throw you off or overtake the line.

Minus that, maybe try putting her in a round pen or arena to just run around before a ride? Because she clearly sounds like she has a lot of excess to burn and once she tires, she's an angel. But again, not a riding expert here.

What about putting her on some sort of chill supplement? Like Chill?

Sudden 180* behavior shifts to me scream that either the horse has an underlying injury that hasn't been diagnosed (did you vet her before purchase?) or that they were previously drugged/supplemented in such a way that the behavior or pain was not showing until the drugs wore off. Definitely recommend a vet, saddle fitter, etc. I know some horses get hotter/more reactive in the winter (#winterzoomies) but that sounds pretty extreme imo.

Equ, she was trained as a PLEASURE horse. Like robotic, desired lope equivalent speed of a normal horses walk, slooooow. Not the trail kind of pleasure lol flashy pleasure. She could possibly make a trail horse. She doesn’t have any back sensitivity from what I can see and I’ve had myself as well as my dad and the friend who rode her put pressure all down her neck, back, butt and tail. We checked her legs, nothing bad. Feet are in good shape. She’s just crazy all of a sudden. The food I can’t get is called MF Easy keeper or something, and there’s one other similar food, but it’s like 25$ a bag and it seems to be about the same nutritional value as the strategy and I don’t want to get her hot again. She’s doing great on the pelleted feed my other horses are on. Maintaining weight and not gaining any more than she needs lol. 

Sodenote: both her babies are beautiful cutting horses somewhere in Texas now and I will be getting a baby from her before I decide to get rid of her because I want a cow horse so bad I can’t stand it lol.

blitzen, i might have to look into some calming supplements when I start working with her again  I’ve never had a horse so hotheaded so I don’t know the good ones! Lol

Mero, I don’t believe it was drugging or supplementation that had her doing so good before as I had her for almost two months before this happened and rode her several times without lunging her beforehand with no issues. I also don’t believe the woman i got her from would’ve let her daughter ride her if she had to be drugged beforehand. I did not vet her before purchase as she was 3 hours away from my hometown and almost 4 from my vet, but she shows 0 sign of injury or pain other than the one incident I had. Some days when I go to her pasture she’s a big ooey gooey baby that just wants to be loved and others she’s the spawn of Satan. I’ve been in the horse game for 23 years and have had a mare that had underlying pain from her foaling the year before I got her, as well as experience from volunteering with my vet when I was younger and a beyond normal interest for the art of equine chiropractic work (which I learned was a thing for the mare I used to have lol) the saddle fits her well and it’s the same saddle/blanket/bridle/bit I’ve used every time before. My dad checked the tack when he unsaddled her and said everything was clean and in fine condition. I have no clue what her deal was, but someone I used to ride for told me if she’s been  trained as a pleasure horse then she’s essentially been trained to buck if not lunged for at least 30-45 minutes before being ridden. I just don’t know if that’s the case since she’s never gone bucking Bronc on me before? It had been maybe 2 weeks since the last time I’d ridden he but I’d gone that long without riding her before so I’m not sure. It’s definitely not winterzoomies it wasn’t even kind of cold when this happened. I think she’s just evil tbh. 

We had a little pony mare at my volunteering who could be rather b-wordy and we used to give her this:

https://greenhawk.com/wdItemDesc.asp?strilhID=Web&strmdNumber=SUV3504&stricSKU=SUV3504

Could it be some sort of anxiety/fear she's demonstrating? or an ulcer?

I’m thinking no on the ulcers because the feed i was giving her at the time had a specific supplement added to prevent and cure ulcers, I mean I guess it could be the supplement didn’t help but I’ve heard nothing but good reviews about it. I guess it could be an anxiety thing but she showed no sign of fear or agitation. Just blew up the second I got into the saddle. 

Pleasure/trail, whatever she was in the past, this is not normal nor typical traits for a horse in that discipline. Have you had Tottie looked over by a veterinary university in a full run-down for a possible brain tumor? My father broke a horse - named All That Class or something - who had similiar temperments as your mare. She was completely sound and walked/worked fine. On one day she'd be the biggest puppy and would roll all over you. The next she was the spawn of Satan trying to crush your skull with no warning (and did crush my father in fact. He had 6 broken ribs, a broken hip, and a dislocated shoulder). They diagnosed the tumor, took it out, and the horse turned out great afterwards. If you have already done this and it's not that, then what about hormones? There was another mare I knew that was always the ultra b-word when she was in estrus, but once she was on birth control, she was perfect, then spayed her a few months later. She became the best mare after that.

Serious question: Do her babies in Texas have good temperments? Have you heard if they've posed similar issues as their mother? If they're good tempered and are good working horses, what stallion(s) did they book? Because if she is this looney, this could easily pass down to the offspring, but if the father was a good temper and made the babies good, I'd be seriously re-booking one of the stallions if you absolutely must get a foal from her.

I really have no input on this, but Strategy made my very level headed, kind, loving, mare into something that was equivilent to a ferral animal lol. 

As for her outbursts, is/was she in heat? I'm thinking if she is in heat and something flares up during her heat cycle it could cause pain. 

I mean this in the most helpful way possible, but you need to have her checked by a vet. This is dangerous and she could have hidden pain or kissing spine or a neurological issue. It's not normal for a horse to suddenly shift behavior like this, it's not a mare thing, it's a horse with a problem thing.

I've had a dangerous horse and I held on for a very long time, but I'm lucky I wasn't injured much worse than I was and that she never managed to hurt anyone else. She got increasingly unhinged with age and I had to have her euthanized eventually.

It could be that she's sort of a hothead already and she was getting too much energy from her feed and maybe even was in heat, but still.

There are so many wonderful horses out there, and hopefully she is one hidden under an easily fixable issue, but please don't break something you can't fix in the process. Be safe!

I'm  going to have to side with everyone saying that you should check into things with a vet. Just because you didn't do a PPE you actually cannot say that there's not something potentially physically wrong with her. Having read the comments I see that you had her two months before she freaked out vs. a couple of days so I'm going to agree that I don't think the  issues that she was drugged at first. But having such a drastic change in behavior isn't normal, it honestly steams like there's some sort of issue somewhere that needs to be found. Please get her completely checked over by a vet before you brush her off as a lost cause. There easily could have been something that wasn't diagnosed or wasn't disclosed.

Fun update on the devil mare since i forgot about this post until fairly recently lol (my bad) 

I finally found a way to get ahold of the vet who owned this mare shortly before i did. Turns out, she has bad ulcers (she was on a feed that comes with supplements for ulcers, but some of the other contents of the feed might have been canceling out the medicated supplement lol) and it was suggested that she be put on plain oats & green grass as that's what she tolerated best in the vet's care. 

HOWEVER, the vet said the ulcers (in her opinion) had little to nothing to do with her throwing me off. The mare has just never been one to like being rode lol. She gets bored of it and decides to throw people. She was pulled out of a w/p training program (by the vet who owned her) with the belief they were probably beating on her when she would get bored and throw them. The vet told me she had tried just laid back, light riding in the arena and the mare lost it and started bucking and going off, so she put her out to pasture. She was actually surprised at the number of times I had ridden her before getting thrown. xD SO it was suggested that we just leave her out to pasture and use her as a broodmare cause she makes a wonderful momma, and her past couple of babies were just superb cow horses. She's (hopefully) just been bred to a beautiful blue roan stud I'll have to find a picture of to post here. Just waiting for her 60 day mark to get the vet out to test her and confirm she's pregnant! :D 

Stud: http://prntscr.com/nis64r

mare: http://prntscr.com/niscaq