The Collaborative International Dictionary of English v.0.48:

Ridgeling \Ridge"ling\, n. [Prov. E. riggilt, riggot, ananimal
   half castrated, a sheep having only one testicle; cf. Prov.
   G. rigel, rig, a barrow hog, rigler a cock half castrated.]
   (Zool.)
   A half-castrated male animal.
   [1913 Webster] Ridgepiece

WordNet (r) 3.0 (2006):

ridgeling
    n 1: a colt with undescended testicles [syn: ridgeling,
         ridgling, ridgel, ridgil]