90482 Orcus, usually referred to simply as Orcus, is Pluto's evil twin.
Orcus is a dwarf planet that very nearly mirrors Pluto. While Orcus shares a similar orbit and orbital inclination to that of Pluto, the orbital inclinations mirror each other across the elliptic, their pathways making a large 'X' on their way around the sun, with the result that the planets are rarely near each other. In addition, they are always in the opposite phase of their orbits: Orcus is at aphelion when Pluto is at perihelion, and vice versa. Because of these factors, Orcus is sometimes called the anti-Pluto.
Orcus was discovered on February 19, 2004 by Mike Brown at the California Institute of Technology. It is a plutino, one of the many trans-Neptunian objects locked in a 2:3 orbital resonance with Neptune (the largest of these being Pluto itself). Orcus is surprisingly close to Pluto in a number of aspects:
The name Orcus was chosen because Under the guidelines of the International Astronomical Union (IAU) objects with a similar size and orbit to that of Pluto are named after underworld deities; Orcus was the Etruscan god of the underworld, and the Romans sometimes conflated him with Pluto. Orcus has one large moon, which is named Vanth after the Etruscan psychopomp.
Due to its recent discovery and difficulty in gaining clear observations, the IAU has not yet officially added Orcus to the list of dwarf planets, although it is expected to be recognized as one as more observations come in.