Sources : Chelydros

Isidore of Seville [7th century CE] (Etymologies, Book 12, 3:): The chelydros is a snake that is also known as the chersydros, as if it were cerim, because it dwells both in the water and on land... These make the earth on which they move smoke... But it always proceeds in a straight line, for if it turns when it moves, it immediately makes a sharp noise. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Serpents 8.12): Celidrus is a serpent, as Ysidorus says, which dwells in the waters and on the land. It has its name from the Greek: chiron is earth, and hydros is water. It causes the ground through which this serpent treads to smoke, whence Macer says: The smoking poison breathes out behind it, the earth smokes through which the snake slides. But it always walks in a straight line, for if it twists itself while moving it immediately makes a cracking sound. - [Badke translation/paraphrase]