A category of stance which is physically distinguishable by a 50%-50% weight distribution, Toe-Toe Heel-Heel Alignment of depth, feet separated at or about the  width of the shoulders - for an alignment of width, and both feet positioned flat on the ground - eyes directed toward the Point of Reference.

Comments:

Horse stances are generally considered to be more geared toward training, but can also be used selectively in combat and self-defense.

The standard Attention stance is typically considered a formal stance - used in formal occasions; such as forms, opening / closing class, etc.

Derivation:

The term horse is taken from the stance's likeness to a person riding a horse.