It's grammatical if the words pair of have been omitted. For example, in (1) below the determiner "a" agrees in number with the noun "pair":Originally Posted by fungyf_hk
(1a) Let's make a monkey's pair of eyes.
(1b) Let's make a pair of eyes, a monkey's pair.
The noun "monkey's" functions as an adjective. It modifies "pair of eyes", or "eyes".