Twenty, on leaving Guinea, acquiring horse? (5)
I believe the answer is:
equid
'horse?' is the definition.
(horse is a kind of equid)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for equid that I've seen before include "A mammal of the horse family" , "An ass perhaps" , "A pony, perhaps" , "could be an ass" .)