first name vs praenomen

first name and praenomen both are nouns.

Word	Noun	Adjective	Verb	Adverb
first name	Yes	No	No	No
praenomen	Yes	No	No	No

As nouns, praenomen is a hyponym of first name; that is, praenomen is a word with a more specific, narrower meaning than first name:

Other hyponyms of first name include baptismal name, Christian name.

first name (noun)	praenomen (noun)
the name that precedes the surname	the first name of a citizen of ancient Rome