Bipedal -- that's unusual for horse! (7)
I believe the answer is:
piebald
'horse' is the definition.
'bipedal that's unusual' is the wordplay.
'that's unusual' indicates anagramming the letters.
'bipedal' with letters rearranged gives 'PIEBALD'.
'for' is the link.
(Other definitions for piebald that I've seen before include "(Of horse) patched with two colours" , "Two-toned" , "(Of an animal) marked in two different colours" , "(Of horse) black and white in patches" , "(Horse) mottled with two colours" .)