WordNet sense 1 (a deceitful or treacherous person):

snake in the grass

WordNet sense 2 (a tributary of the Columbia River that rises in Wyoming and flows westward; discovered in 1805 by the Lewis and Clark Expedition):

Snake River

WordNet sense 3 (a long faint constellation in the southern hemisphere near the equator stretching between Virgo and Cancer):

hydra

WordNet sense 4 (limbless scaly elongate reptile; some are venomous):

serpent, ophidian