The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         Their wounded steeds
         Fret fetlock deep in gore.               --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "fetlock":
   arch, bang, bangs, clubfoot, digit, dog, extremity, fleck, flock,
   foot, forefoot, forelock, forepaw, fringe, harefoot, heel, hoof,
   instep, pad, pastern, patte, paw, pedal extremity, pedes, pes,
   pied, pug, quiff, sole, splayfoot, toe, tootsy, trotter, tuft,
   ungula