Acid Horse is a song from Silent Hill: Origins. The song is track 10 in the game's soundtrack and was composed by Akira Yamaoka. Eerie sounds play through the track, with piano and what sounds like a slow roll of a train going down the tracks.
It plays at several points in the game: after Travis Grady solves the Flauros puzzle, when he meets Dahlia Gillespie outside of Alchemilla Hospital, on the way to Green Lion Antiques, and during the final boss fight.