Being a Furry and a Brony, I can say that yes there are sites dealing with ponies. In fact there are several sites that are for fandom news, art, fanfiction, role playing and conventions. I'm not sure Vgmaster9 what you are asking. If you're asking why ponies are on furry sites like here, my response is why not? The furry fandom is probably the most open fandom for the expression of art. We have everything from normal looking animals that happen to talk to cat girls from Anime so ponies would definitely fit into that spectrum of acceptance as they are animals with many human traits.
As far as groups dedicated to certain species of furries, I know FA doesn't have many groups but on DeviantArt there are groups like these for feline, canine, equine or any other furry type.
Well the kind of fandom I'm speaking on doesn't have to exclusively involve ponies/horses. It can involve other animals like deer, bovids, and other different hoofed animals, as well as legendary creatures like dragons, griffons, minotaurs, etc. They'd all just have the design aesthetics of MLP.