Felis onca vs big cat

Felis onca and big cat both are nouns.

Word	Noun	Adjective	Verb	Adverb
Felis onca	Yes	No	No	No
big cat	Yes	No	No	No

As nouns, big cat is a hypernym of Felis onca; that is, big cat is a word with a broader meaning than Felis onca:

Felis onca: a large spotted feline of tropical America similar to the leopard; in some classifications considered a member of the genus Felis
big cat: any of several large cats typically able to roar and living in the wild

Other hypernyms of Felis onca include cat.

Felis onca (noun)	big cat (noun)
a large spotted feline of tropical America similar to the leopard; in some classifications considered a member of the genus Felis	any of several large cats typically able to roar and living in the wild