The dutch astronomer Gerald Kuiper theorized about the KBOs back in 1951 and they were discovered one after the other from 1992..
Kuiper had comets in mind.
Triton also came from the KBO area (its larger than Pluto) and it must have had a companion in order to explain how it had been captured by Neptune.
I read a paper suggesting no very large moon (earth sized) can exist around gas giants. They should at most be 1/10,000 the planet's mass. If we find exceptions (like our Moon, Triton) then they must have been captured.