horsefly vs horse fly

horsefly and horse fly both are nouns.

Word	Noun	Adjective	Verb	Adverb
horsefly	Yes	No	No	No
horse fly	Yes	No	No	No

As nouns, horsefly and horse fly are synonyms defined as:

horsefly and horse fly: large swift fly the female of which sucks blood of various animals

Other synonyms of horsefly include cleg, clegg.

horsefly (noun)	horse fly (noun)
large swift fly the female of which sucks blood of various animals	large swift fly the female of which sucks blood of various animals
winged fly parasitic on horses