Crossword Genius

Drug mule's relative? (5)

Ross

I believe the answer is:

horse

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This might be a double definition.

'drug' is the first definition.
(horse is a kind of drug)

'mule's relative?' is the second definition.
The answer and definition can be both animals as well as being singular nouns.
Maybe they are linked in a way I don't understand?

Can you help me to learn more?

(Other definitions for horse that I've seen before include "Nag, hack" , "Mount" , "Cavalry - gymnastic apparatus" , "Animal featuring in the sport of kings" , "Eg, mare" .)

I've seen this clue in The Guardian.
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