A person typically has hair under a pony tail. Under a pony's tail is their anus which is where they defecate from.
Looks like the tail of a well-groomed pony.
it looks like a horses tail, obviously.
the Chinese king had a pony tail too
put it in a pony tail
up in a pony tail
brushing the mane and tail of your horse/pony
it's named a ponytail because it looks like the tail of a pony. And another thing: you need to have a ponytail when you're riding on horses for your safety.