Well, your name is an indicator of being a hater- but I will look past that to explain a few things from an open mind to an inquisitive mind.
There is quite a large fanbase that the Brony community just so happens to border on, call the Furry Community. Furries are various fans of Anthropomorphic Animals (such as the picture you have just seen). I myself admit to liking that kind of art work.
To you next question on who the hell wants to fuck a pony- I'd say zoophiles. But I assume you are linking the whole 'ponies with boobs' thing. There is a sub-community of furries (known as furfags and furverts) who are actually into furry-based hentai and the like.