The Collaborative International Dictionary of English v.0.48:

Pluto \Plu"to\ (pl[=u]"t[-o]), pr. n. [L., fr. Gr. Plou`twn.]
   1. (Class. Myth.) The son of Saturn and Rhea, brother of
      Jupiter and Neptune; the dark and gloomy god of the Lower
      World.
      [1913 Webster]

   2. The ninth planet of the Solar System, the smallest (5700
      km radius) and most distant from the sun. The suggestion
      has been made that it more closely resembles a large close
      comet than a planet. Its orbit has an eccentricity of
      0.248, larger than that of any other planet; it varies
      from 4.44 to 7.37 billion km distance from the sun.
      [PJC]

            Pluto is an oddball among its eight sister planets.
            It's the smallest in both size and mass, and has the
            most elliptical orbit. It moves in a plane tilted
            markedly away from the other planets' orbits.
            Moreover, Pluto is the only planet made almost
            entirely of ice.                      --Ron Cohen
                                                  (Science News,
                                                  Feb. 27, 1999,
                                                  p. 139)

   Pluto monkey (Zool.), a long-tailed African monkey
      (Cercopithecus pluto), having side whiskers. The general
      color is black, more or less grizzled; the frontal band is
      white.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Pluto
    n 1: a cartoon character created by Walt Disney
    2: (Greek mythology) the god of the underworld in ancient
       mythology; brother of Zeus and husband of Persephone [syn:
       Pluto, Hades, Aides, Aidoneus]
    3: a small planet and the farthest known planet from the sun; it
       has the most elliptical orbit of all the planets; "Pluto was
       discovered by Clyde Tombaugh in 1930"

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "Pluto":
   Agdistis, Aides, Aidoneus, Amor, Aphrodite, Apollo, Apollon, Ares,
   Artemis, Ate, Athena, Bacchus, Cerberus, Ceres, Charon, Cora,
   Cronus, Cupid, Cybele, Demeter, Despoina, Diana, Dionysus, Dis,
   Dis pater, Earth, Erebus, Eros, Gaea, Gaia, Ge, Great Mother,
   Hades, Hel, Helios, Hephaestus, Hera, Here, Hermes, Hestia, Hymen,
   Hyperion, Jove, Juno, Jupiter, Jupiter Fidius, Jupiter Fulgur,
   Jupiter Optimus Maximus, Jupiter Pluvius, Jupiter Tonans, Kore,
   Kronos, Loki, Magna Mater, Mars, Mercury, Minerva, Minos, Mithras,
   Momus, Neptune, Nike, Olympians, Olympic gods, Ops, Orcus, Osiris,
   Persephassa, Persephone, Phoebus, Phoebus Apollo, Poseidon,
   Proserpina, Proserpine, Rhadamanthus, Rhea, Satan, Saturn, Tellus,
   Uranus, Venus, Vesta, Vulcan, Zeus, asteroid, inferior planet,
   major planet, minor planet, planet, planetoid, secondary planet,
   solar system, superior planet, terrestrial planet, wanderer