My mum's favourite horse was a total bitch who hated everyone and everything except for playing polocrosse (she was a polocrosse horse). When she was on the field she'd do everything to win, including keeping a stupid rider properly seated no matter how bad at staying on they were, but the instant she was off the field all bets were off. Her favourite tricks included:
breathing in deep when being saddled for anything that wasn't a polocrosse game, so that the saddle straps will be loose and the rider will fall on their arse. This is a favourite trick of bitch horses and most horse people will check for it so it tends to fail.
biting people. Her name was Nipper, because she liked to bite people.
Doing the breathing trick, waiting for you to correct her and tighten the saddle, and biting you when you were distracted by that
stepping on your foot when you were leading her or standing with her. This wasn't being clumsy. She would step on peoples' feet on purpose.
standing behind you with her head over your shoulder watching stuff, then deciding she wants to watch over your other shoulder. When a horse wants to do this, they just lift their head up over yours; easy. Nipper would lift her head up over yours, then dip her head down halfway through to hit you on the head with her jaw.
chewing the fuck out of everything
chasing dogs, other horses, and small children (not in a 'let's play together' way, she found their fear amusing)
enticing people to hand feed her and then biting the hand that feeds her
She was always very careful never to hurt anyone more than a bruise but boy did she love giving those bruises. If you were in real danger (such as being pressed between two horses and risking being trampled), she would use her own body to protect you. And then bite you. But she'd play these stupid games constantly and then as soon as she was on the field it was like 'me and my rider are best friends, we are a TEAM that will WIN THIS GAME' and was totally trustworthy until the chukka was over. (Between chukkas, she was not trustworthy. I'd keep her warm between chukkas and she'd resent me because she wanted to be out on the field playing with the other horses, not doing something silly and pointless like 'take a few minutes at a slower pace so your stupid horse heart doesn't explode'. Between chukkas was prime Step On Derin's Feet time.)