WordNet (r) 3.0 (2006):

snake in the grass
    n 1: a deceitful or treacherous person [syn: snake, snake in
         the grass]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "snake in the grass":
   Benedict Arnold, Brutus, Judas, Judas Iscariot, Quisling,
   abomination, archtraitor, atrocity, bad, bane, befoulment,
   betrayer, blight, cockatrice, conniver, conspirator, conspirer,
   coral heads, corruption, crying evil, damage, defilement,
   despoliation, destruction, detriment, double agent, double-crosser,
   double-dealer, evil, grievance, harm, havoc, hurt, ill, infection,
   informer, injury, intrigant, intriguer, ironbound coast, ledges,
   lee shore, machinator, mischief, outrage, pitfall, plotter, poison,
   pollution, quicksands, quisling, rat, rockbound coast, rocks,
   sandbank, sandbar, sands, schemer, serpent, shallows, shoals,
   snake, the worst, timeserver, toxin, traitor, treasonist, trimmer,
   turncoat, undercurrent, undertow, venom, vexation, woe, wrong