Beast that is cross — bull initially confined (4)
I believe the answer is:
ibex
But I can't explain it at all! Could you help me understand?
(Other definitions for ibex that I've seen before include "Sure-footed type" , "Pyrenean mountain goat" , "Wild goat with double curved horns" , "Mountain goat with curving horns" , "Wild goat of North Africa" .)