king of beasts vs big cat

king of beasts and big cat both are nouns.

Word	Noun	Adjective	Verb	Adverb
king of beasts	Yes	No	No	No
big cat	Yes	No	No	No

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

king of beasts: large gregarious predatory feline of Africa and India having a tawny coat with a shaggy mane in the male
big cat: any of several large cats typically able to roar and living in the wild

Other hypernyms of king of beasts include cat.

king of beasts (noun)	big cat (noun)
large gregarious predatory feline of Africa and India having a tawny coat with a shaggy mane in the male	any of several large cats typically able to roar and living in the wild