Feral horse has to, nag is different (7)
I believe the answer is:
mustang
'feral horse' is the definition.
(I've seen this before)
'has to nag is different' is the wordplay.
'has to' becomes 'must' (I've seen this before).
'is different' indicates an anagram.
'nag' anagrammed gives 'ang'.
'must'+'ang'='MUSTANG'
(Other definitions for mustang that I've seen before include "Feral American horse" , "Trotter" , "one running wild" , "creature" , "Small feral horse of the US" .)