Have I ever told you that my trainer's horse refuses to pee while OUTSIDE? That really blows my mind.
I know horses, mares in particular, who seem to like to wait and pee as soon as they get in the stall. Our two do that, or they come in from pasture and pee in the lean-to. Makes us crazy.
I think mares like to pee in deep bedding so it doesn't splash on their legs :~D
My husband's gelding won't urinate if he's saddled, whether or not he has a rider. Then, most of the mares go into their stalls to do it.