horse-pistol vs horse pistol

horse-pistol and horse pistol both are nouns.

Word	Noun	Adjective	Verb	Adverb
horse-pistol	Yes	No	No	No
horse pistol	Yes	No	No	No

As nouns, horse-pistol and horse pistol are synonyms defined as:

horse-pistol and horse pistol: a large pistol (usually in a holster) formerly carried by horsemen

horse-pistol (noun)	horse pistol (noun)
a large pistol (usually in a holster) formerly carried by horsemen	a large pistol (usually in a holster) formerly carried by horsemen