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]