Rex vs sovereign

Rex and sovereign both are nouns.

Word	Noun	Adjective	Verb	Adverb
Rex	Yes	No	No	No
sovereign	Yes	Yes	No	No

As nouns, sovereign is a hypernym of Rex; that is, sovereign is a word with a broader meaning than Rex:

Other hypernyms of Rex include adult male, man, crowned head, monarch.

Rex (noun)	sovereign (noun)
a male sovereign; ruler of a kingdom	a nation's ruler or head of state usually by hereditary right

Rex (adjective)	sovereign (adjective)
	greatest in status or authority or power
	(of political bodies) not controlled by outside forces