I saw a movie on TV when I was younger it was a war film about an american army unit sent to build a runway on an island. When there they find a horse and the horse befriends them and they work together, he carries water to the men as they work and helps with heavy work and they look after the horse. When they have to leave they are told they have to leave the horse behind. But the horse swims out after the boat and the commanding officer says to lower the tail board as it doesn't seam to be closed properly allowing the horse time to climb on board.

Gallant Bess (http://www.tcm.com/tcmdb/title.jsp?stid=75901)