feline vs true cat

feline and true cat both are nouns.

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

As nouns, true cat is a hyponym of feline; that is, true cat is a word with a more specific, narrower meaning than feline:

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

Other hyponyms of feline include cat, big cat, cat.

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

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