byname vs sobriquet

byname and sobriquet both are nouns.

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

As nouns, byname and sobriquet are synonyms defined as:

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

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

byname (noun)	sobriquet (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)