splint bone

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of two small metacarpal or metatarsal bones in horses or related animals.

from The Century Dictionary.

• noun In anatomy:
• noun In farriery, a splint; one of the reduced lateral metacarpals or metatarsals of the horse, closely applied to one side of the back of the cannon-bone, or middle metacarpal or metatarsal. See cuts under cannonbone, Perissodactyla, pisiform, and solidungulate.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a rudimentary metacarpal or metatarsal bone on either side of the cannon bone in the leg of a horse or related animal