Billy the Kid vs Billie the Kid

Billy the Kid and Billie the Kid both are nouns.

Word	Noun	Adjective	Verb	Adverb
Billy the Kid	Yes	No	No	No
Billie the Kid	Yes	No	No	No

As nouns, Billy the Kid and Billie the Kid are synonyms defined as:

Billy the Kid and Billie the Kid: United States outlaw who was said to have killed 21 men (1859-1881)

Other synonyms of Billy the Kid include Bonney, William H. Bonney.

Billy the Kid (noun)	Billie the Kid (noun)
United States outlaw who was said to have killed 21 men (1859-1881)	United States outlaw who was said to have killed 21 men (1859-1881)