There are two types of pony tails. The ones attached to a pony (or a horse), are natural. In other words they were not invented. That's just the way these animals were created. Almost animals have tails, and they are all shaped differently yet they are still tails.
If you're referring to the way a person ties their hair behind their head, which is called a "pony tail", I doubt this question has an answer. It's not really an "invention". It likely goes way back in history, and was just a method used to keep a persons hair from getting in their face while doing work or other activities. It was likely given the name "pony tail" because it looks like the tail on a horse or pony, and pony breeds such as the Shetland pony, have very thick tails. This is likely the reason they used the word "pony". I doubt anyone will ever know who gave it that name. It's just one of those things where someone gives something a name, and that name carries on from person to person.
A person typically has hair under a pony tail. Under a pony's tail is their anus which is where they defecate from.
cuz the person who invented the word probally thought it looked like a PONYS TAIL
Looks like the tail of a well-groomed pony.
pony tail
it looks like a horses tail, obviously.
the Chinese king had a pony tail too
brushing the mane and tail of your horse/pony
up in a pony tail
put it in a pony tail
Emery
pony tail
bony bail