coffin bone

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

• noun The bone enclosed inside a horse's hoof.

from The Century Dictionary.

• noun The last phalanx of a horse's foot; the distal phalangeal bone. See hoof.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The bottommost bone in the equine leg, encased by the hoof capsule.