Hydra

The hydra was first written in greek and roman mythology. It was described as a huge aquatic serpentine like monster with many heads and poisonous stench. If you chopped one head off, it regrew two more. It dwelled in the lake of Lerna, which was said to be a gate to the underworld. It's said that the monster was slain by Hercules, he chopped off its heads and burned its neck off with the help of his nephew Lolaus. In d&d, Hydras are described to have a more crocodilian appearance, as well as not having poisonous breath.