byname vs soubriquet

byname and soubriquet both are nouns.

Word	Noun	Adjective	Verb	Adverb
byname	Yes	No	No	No
soubriquet	Yes	No	No	No

As nouns, byname and soubriquet are synonyms defined as:

byname and soubriquet: a familiar name for a person (often a shortened version of a person's given name)

Other synonyms of byname include cognomen, moniker, nickname, sobriquet.

byname (noun)	soubriquet (noun)
a familiar name for a person (often a shortened version of a person's given name)	a familiar name for a person (often a shortened version of a person's given name)