The Collaborative International Dictionary of English v.0.48:

Ridgel \Ridg"el\, n. (Zool.)
   Same as Ridgelling.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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