There's horseisle 1 and horseisle 2, I would really recommend the first one!
Ever since the release of HI2, more than half of our players have moved to HI2, so new players are always welcome in HI1! Both are way different than howrse, though. Both are virtual worlds where you can catch and trade horses, and there is no breeding except in HI2.
HI1, unlike HI2, is free to play with the added benefits if you do decide to pay, but it is totally playable and still fun without paying. In HI2, you are VERY limited. There's quests involved, mini games & chatting. It takes a bit of getting used to, but there's many helpful people. If you make an account in HI1, you'll have to choose what server to join; whichever you choose, you pretty much have to stick to that one unless you want to start anew on another server, since your player/profile can not be transferred. Now, you can join whichever you like, of course! But you can find me on HI1, chestnut server, and i'll be there to help you! Just send me a pm here if you wish to give the game a shot.
horseland.com and horseeden.com are both horse games. I don't play on either one any more though, so I'm not sure if they're any good these days. They were decent enough when I tried them though.