A snake, or serpent, is a legless reptile of the suborder serpentes. They have scaled skin and some varieties are poisonous.
The snake was regarded as a symbol for slyness and evil by Christians during the Middle Ages due to its role as the tempter in the Biblical account of Adam and Eve. A legend of St Patrick tells of him driving all snakes out of Ireland. Of course, there may have been none there to start with.
The snake was believed to be related to the dragon and thus both were often described as sharing certain traits.