Billie the Kid vs Billy the Kid

Billie the Kid and Billy the Kid both are nouns.

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

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

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

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

Billie the Kid (noun)	Billy 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)