Well Pluto is ONE example of trillions!!!! of
icy bodies in the KUIPER COMET belt. This is a
region from beyond the orbit of Neptune extending
to about 100 Astronomic Units from the Sun of a
belt of comets. Pluto is just one of the bigger of
these objects.
Answer 2:
I don't think there is an answer that. The matter
that made Pluto didn't make it large. Why there
aren't any larger comet-like planets in the solar
system might be a good question, though.
