true cat vs feline

true cat and feline both are nouns.

Word	Noun	Adjective	Verb	Adverb
true cat	Yes	No	No	No
feline	Yes	Yes	No	No

As nouns, feline is a hypernym of true cat; that is, feline is a word with a broader meaning than true cat:

true cat: feline mammal usually having thick soft fur and no ability to roar: domestic cats; wildcats
feline: any of various lithe-bodied roundheaded fissiped mammals, many with retractile claws

Other hypernyms of true cat include felid.

true cat (noun)	feline (noun)
feline mammal usually having thick soft fur and no ability to roar: domestic cats; wildcats	any of various lithe-bodied roundheaded fissiped mammals, many with retractile claws

true cat (adjective)	feline (adjective)
	of or relating to cats