A horse boot (10) includes a shell (12) for a hoof and a gaiter (22) for the pastern. The shell has a sole (14), an upper (16), a tongue (18), and an elastic strap (60) for tightening the upper and the tongue around the hoof of a horse. The gaiter (22) is releasably attached to the upper so as to extend around the rear of the shell and can be wrapped around the pastern (P) of the horse immediately above the hoof's hairline (C) and below the fetlock to function as a barrier against the entry of debris into the shell. The gaiter provides an additional anchor to retain the shell in place under extreme conditions of use. The releasable connection to the shell (12) allows replacement of the gaiter (22) when worn out.