According to The Dictionary
"catch colt n ... chief
, Inland Nth See Maps
1 The offpsring of a mare bred accidentally.
-41 Cassidy !
WI Atlas csWI,, Catch-- Colt that was unintentionally bred. 1958 AmSp 33.271, Ketch-colt
. An offspring obviously not from the herd sire.
2 By ext: a child born out of wedlock."