A Plutoid (or Ice Dwarf, a term more common within science) is a Pluto-like Dwarf Planet. As such, it is icy, round, a Trans-Neptunian Object (orbits beyond Neptune) and a KBO (Kuiper Belt Object)
In addition to Pluto, the known examples are Eris, Haumea, and Makemake.
A similar term, Plutino is defined not by its composition, but by its orbit: like Pluto, having an Orbital Resonance with Neptune. Other than Pluto, they are all small objects. Estimates are there are thousands of Plutinos.