A Pegasister is a female person who is a fan of the TV series named My Little Pony: Friendship is Magic, typically at the age of 13 or older. If you're male on the other hand, you would be known as a Brony. The plural for this term is Pegasisters, and the plural for the male term is Bronies.
A new, but not yet official, term for females who watch and enjoy My Little Pony: Friendship is Magic. They are mostly in the same age group as your ordinary brony. There is no different brohoof for pegasisters, so it's pretty much the same as being a female brony.
Related words: brony, brohoof, pegasis, My Little Pony