Dun horse? (3)
I believe the answer is:
nag
I believe this is a double definition.
'dun' is the first definition.
(thesaurus)
'horse?' is the second definition.
(term for an old horse)
(Other definitions for nag that I've seen before include "Constantly scold - small horse" , "Scold; inferior horse" , "its head to name the pub" , "Complain continuously like old horse" , "Old horse or shrewish wife" .)