Many cities and kingdoms on Equestria are far away from each other and barely have rest stops on roads, so ponies need to relieve themselves outside: behind a bush or tree or peeing straight on the grass or ground.

Villains don't escape from this situation, like Opaline and Misty (very years before meeting Mane 5) always had pee breaks while they were far from Opaline's castle.

Luckly both villains knew to pee standing up, very convenient for both, specially for Misty.