I had one who would pee on the way to the arena at home almost every time.
Another one would pee in the crossties after a ride. I had to make sure to untack him in the wash rack instead of in the aisle, especially when at my trainer’s barn. (Very embarassing for him to pee in her aisle.) If he didn’t pee in the washrack after a lesson, he usually peed in the trailer on the way home. We don’t go to lessons anymore (he’s basically retired at 23), but he still pees in the washrack at home after a ride.
I knew a lady who taught her horses to pee and poop directly into a muck bucket on command!