I'm not sure whether the colocation "cat's eyes" is plural or singular.
Should it be:
"I have seen a cat's eyes." or
"I have seen cat's eyes."
I think it should be without "a" as the eyes are plural...
You may be interested to know that there is an idiom here: the little reflective rubber-mounted things that mark lanes in a road are called "cat's eyes" (Cat's eye (road) - Wikipedia, the free encyclopedia ). They were invented by someone who had seen a cat's eyes; so, as well as teia's "I have seen some cats` eyes' (referring to feline vision) you can say "I have seen ['some' - optional] cat's eyes". (The stress is different: in the figurative usage, the stress is always on the first syllable.)