A horse boot (10) includes a shell (12) and a gaiter (22). The shell has a sole (14) and an upper (16) with a vertical slit (18). The gaiter (22) is releasably attached to the heel of the upper and is wrapped around the pastern (P) above the hoof's hairline (C) and below the fetlock to anchor the shell to the pastern and function as a barrier against debris. The gaiter includes a portion of two-way stretch material (60) that makes it possible to fold it out of the way for ease of installation. The process is further facilitated by the slit (18) that allows spreading of the shell. By shaping the upper (16) so that it presses against the hoof (H), the boot is advantageously held in place using only the gaiter anchored to the heel of the boot and strapped to the pastern of the horse.