The Berithir

Berithirs are serpent / dragon creatures tied to the Fuath of the Scottish Highlands. Their name means in Scottish Gaelic serpent, lightning, and thunderbolt. In some sources they are referred to as beithir-nimh ("venomous serpent") and nathair ("serpent" and "adder").
The beithir is described as "the largest and most deadly kind of serpent", or as a dragon (but without certain typical draconic features such as wings or fiery breath). It dwells in mountainous caves and corries (valleys) and is equipped with a venomous sting. If a person is stung by the beithir then they must head for the nearest body of water such as a river or loch. If they can reach it before the beithir does then they are cured, but if the monster reaches it first then the victim is doomed. Another cure for the sting is water in which the head of another snake has been placed. The beithir is considered one of the fuath, a general term for various monsters and spirits associated with water. It is said that if a normal snake is killed then the head must be separated a proper distance from its body and destroyed. Otherwise, both parts will come together and the snake will return to life as a beithir. The beithir was said to be sighted on summer nights when lightning strikes occurred. The languages it speaks, if any, or at least understands is Scottish Gaelic (and Irish), Brythonic, and Draconic, as well as Sylvan. With the Draconic it is likely that it's ancestors are related to the dragons of old.