A mythical king - or a type of macaque (6)

I believe the answer is:

rhesus

(Other definitions for rhesus that I've seen before include "The Rh in "Rh factor"" , "Kind of monkey (or factor)" , "Small, long-tailed monkey" , "Kind of monkey or blood group" , "Primate" .)

I've seen this clue in The Guardian.

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