WordNet (r) 3.0 (2006):

clop
    n 1: the sound of a horse's hoofs hitting on a hard surface
         [syn: clip-clop, clippety-clop, clop, clopping,
         clunking, clumping]
    v 1: make or move along with a sound as of a horse's hooves
         striking the ground [syn: clop, clump, clunk,
         plunk]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "clop":
   bump, chink, click, clink, clump, clunk, crump, drub, dull thud,
   flick, flump, footfall, footstep, hippety-hop, hoofbeat, hop, jump,
   pace, pad, pat, patter, pitapat, pitter-patter, plump, plunk, pop,
   rap, skip, stamp, step, stomp, stride, tap, thud, thump, tick,
   tinkle, trample, tread, tunk