true cat vs Felis catus

true cat and Felis catus both are nouns.

Word	Noun	Adjective	Verb	Adverb
true cat	Yes	No	No	No
Felis catus	Yes	No	No	No

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

true cat: feline mammal usually having thick soft fur and no ability to roar: domestic cats; wildcats
Felis catus: any domesticated member of the genus Felis

Other hyponyms of true cat include domestic cat, Felis domesticus, house cat, wildcat.

true cat (noun)	Felis catus (noun)
feline mammal usually having thick soft fur and no ability to roar: domestic cats; wildcats	any domesticated member of the genus Felis