H.O.R.S.E. stands for Holdem, Omaha 8/Better, Razz, Seven Card Stud High, Eight/Better Seven Card Stud, and is currently available in tournament and sit&go format.
HORSE starts with HoldEm, and after 10 minutes the game switches to Omaha 8. Every ten minutes thereafter it switches to the next game, and then repeats itself. This is a thrilling game, and only available at Full Tilt Poker.