The Naga, of Hindu mythology were vile creatures, who's purpose seemed to be just to bite other things and kill them with their poison. Seen traditionally as higher beings than humans, there is a nicer, more notable example being Shesha, the naga prince who holds up the Earth. This marked the change in opinion of Nagas. In more modern Hinduism, Nagas are seen as nature spirits, and thus benevolent. They are usually tied to water and treasure and are traditionally snakes that can take human form, rather than a mix of the two.
Also known as the World Serpent, Jörmungandr is a sea serpent born of Loki. Banished to the oceans by Odin, Jörmungandr grew so large that it could encircle the world and hold his tail in his mouth. Once his tail gets free, Ragnarok will begin. Jörmungandr is destined to be the death of Thor, who he will fight in Ragnarok, being slain by Thor but who's poison will kill the god shortly.
The Serpent of Eden
In Christian mythology, the original sin was committed by Eve, after being tricked by a serpent in the Garden of Eden. Once God had found out, he removed the snakes ability to speak, and cast out Adam and Eve and their offspring from the garden and heaven.
The Aztec god of wind and learning, Quetzalcoatl was born of the human Chimalman, who conceived the winged serpent in many ways, depending on the version of the story you read. In Aztec culture, our sun was the 5th one of the earth cycle, and Quetzalcoatl created the 5th age by using the bones of the previous and his own blood. He later commits incest and burns himself alive, creating the morning star.