WordNet (r) 3.0 (2006):

swayback
    adj 1: having abnormal sagging of the spine (especially in
           horses) [syn: dipped, lordotic, swayback,
           swaybacked]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "swayback":
   bowlegs, camelback, cleft palate, clubfoot, crookback, defacement,
   deformation, deformity, disfigurement, flatfoot, freakishness,
   harelip, humpback, hunchback, knock-knee, kyphosis, lordosis,
   malconformation, malformation, misproportion, misshape,
   monstrosity, mutilation, splayfoot, talipes, teratology,
   torticollis, truncation, valgus, wryneck