The Collaborative International Dictionary of English v.0.48:

Cerberus \Cer"be*rus\, n. [L. Cerberus (in sense 1), gr. ?.]
   [1913 Webster]
   1. (Class. Myth.) A monster, in the shape of a three-headed
      dog, guarding the entrance into the infernal regions,
      Hence: Any vigilant custodian or guardian, esp. if surly.
      [1913 Webster]

   2. (Zool.) A genus of East Indian serpents, allied to the
      pythons; the bokadam.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Cerberus
    n 1: (Greek mythology) the three-headed dog guarding the
         entrance to Hades; son of Typhon [syn: Cerberus,
         hellhound]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "Cerberus":
   Aides, Aidoneus, Charon, Cora, Despoina, Dis, Dis pater, Erebus,
   Hades, Hel, Kore, Loki, Minos, Orcus, Osiris, Persephassa,
   Persephone, Pluto, Proserpina, Proserpine, Rhadamanthus, Satan,
   bandog, guard dog, watchdog



The Devil's Dictionary (1881-1906):

CERBERUS, n.  The watch-dog of Hades, whose duty it was to guard the
entrance -- against whom or what does not clearly appear; everybody,
sooner or later, had to go there, and nobody wanted to carry off the
entrance.  Cerberus is known to have had three heads, and some of the
poets have credited him with as many as a hundred.  Professor
Graybill, whose clerky erudition and profound knowledge of Greek give
his opinion great weight, has averaged all the estimates, and makes
the number twenty-seven -- a judgment that would be entirely
conclusive is Professor Graybill had known (a) something about dogs,
and (b) something about arithmetic.