WordNet (r) 3.0 (2006):

Loch Ness monster
    n 1: a large aquatic animal supposed to resemble a serpent or
         plesiosaur of Loch Ness in Scotland [syn: Loch Ness
         monster, Nessie]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "Loch Ness monster":
   Argus, Briareus, Cerberus, Charybdis, Cyclops, Echidna, Gorgon,
   Harpy, Hydra, Medusa, Minotaur, Pegasus, Python, Scylla, Sphinx,
   Talos, Typhon, alevin, benthon, benthos, centaur, cetacean,
   chimera, cockatrice, dolphin, dragon, drake, fingerling, fish, fry,
   game fish, griffin, grilse, hippocampus, kipper, man-eater,
   man-eating shark, marine animal, mermaid, merman, minnow, minny,
   nekton, nixie, ogre, ogress, panfish, plankton, porpoise, roc,
   salamander, salmon, satyr, sea horse, sea monster, sea pig,
   sea serpent, sea snake, shark, siren, smolt, sponge, troll,
   tropical fish, unicorn, vampire, werewolf, whale, windigo,
   xiphopagus, zombie