Source : Webster's Revised Unabridged Dictionary (1913)

Fetlock \Fet"lock\, n. [OE. fetlak, fitlock, cf. Icel. fet pace,
   step, fit webbed foot of water birds, akin to E. foot.
   [root]77. See {Foot}.]
   The cushionlike projection, bearing a tuft of long hair, on
   the back side of the leg above the hoof of the horse and
   similar animals. Also, the joint of the limb at this point
   (between the great pastern bone and the metacarpus), or the
   tuft of hair.

         Their wounded steeds Fret fetlock deep in gore. --Shak.

Source : WordNet®

fetlock
     n 1: the joint between the cannon bone and the pastern [syn: {fetlock
          joint}]
     2: projection behind and above a horse's hoof