I've ridden an old QH mare aged 29 or 30 or so, and sometimes she gets it in her to act like a 2 year old, and either royally misbehave under saddle and decide that everything is out to kill her, or go tearing around the pasture turning on the afterburners with the rest of the mares. Usually after she does that her back is sore for a couple of days because she acts like a bronc. :-)
She has all the young horses running for cover when she does her Queen of the Night act . . . she shrieks and bucks and rears up on her hind legs and chases them around with her neck weaving like a snake.
If you walk into the pasture while she's doing all this, she comes back down to earth and minces towards you as prim as a debutante, "What? Did you want to speak to me?"