I have a question..how are you going to do this? Howrse was built with an html script, something not many people know how to do. Ifyou know how, thats awesome, but chances are you will have to hire someone, and they probably wont work for free.
Another thing, howrse needs money to stay alive, lol. They need to pay their artists, scirpters, staff, the people at owilent, the web address even costs money. Thats why they make passes so special.
Im not trying to put you down. Its just, creating a site is a lot of work. Ecspecially one like howrse. Ive created sites before, and its very hard.
However, I would totally join! Good luck! :D