Twenty, on leaving Guinea, acquiring horse? (5)

I believe the answer is:

equid

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'horse?' is the definition.
(horse is a kind of equid)

I can't explain the remainder of the clue.

(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" .)

I've seen this clue in The Times.

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