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.