Rex vs king

Rex and king both are nouns.

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

As nouns, Rex and king are synonyms defined as:

Other synonyms of Rex include male monarch.

Rex (noun)	king (noun)
a male sovereign; ruler of a kingdom	(chess) the weakest but the most important piece
	one of the four playing cards in a deck bearing the picture of a king
	a checker that has been moved to the opponent's first row where it is promoted to a piece that is free to move either forward or backward
	a male sovereign; ruler of a kingdom
	a very wealthy or powerful businessman
	a male competitor who holds a preeminent position
	the husband or widower of a queen
	preeminence in a particular category or group or field