Howrse is an online persistent tamagotchi-type game which is all about breeding horses and ponies. It is an entirely browser-run game, with no download required. The game is maintained and updated by Owlient.
The game itself is very time-gated in the first month after registering. For the first ten days, you can only feed and care for the horse you chose at registration. After 10 days, you can start selling and bidding for horses at the Auction. After 20 days you can create your own equestrian centre and have multiple horses. After 30 days you are eligible to participate in Direct Sales, in which you can finally instantly buy and sell horses without waiting and bidding in the auctions.
The game features over 50 different breeds of horses; most of these are real-life ones, but some of them such as Unicorn and Pegasus are from fantasy. You interact with your virtual horses on a simple web-based interface using simple 2D graphics.
If you like social websites and games centered around horses, Howrse is for you. It can be played on any PC which has internet connection, but beware: it can be very addictive after getting through the first 20-30 days or so, when the game's possibilities really open up.