The Collaborative International Dictionary of English v.0.48:

Sway-backed \Sway"-backed`\, a.
   Having the back hollow or sagged, whether naturally or as the
   result of injury or weakness; -- said of horses and other
   animals.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "swaybacked":
   bandy, bandy-legged, blemished, bloated, bowlegged, club-footed,
   defaced, deformed, disfigured, dwarfed, flatfooted, grotesque,
   ill-made, ill-proportioned, ill-shaped, knock-kneed, malformed,
   marred, misbegotten, misproportioned, misshapen, monstrous,
   mutilated, out of shape, pigeon-toed, pug-nosed, rachitic, rickety,
   simous, snub-nosed, stumpy, talipedic, truncated