king vs Rex

king and Rex both are nouns.

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

As nouns, king and Rex are synonyms defined as:

Other synonyms of king include male monarch.

king (noun)	Rex (noun)
(chess) the weakest but the most important piece	a male sovereign; ruler of a kingdom
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