The cerastes is the most flexible of all serpents, so flexible that is seems to have no spine. It has either two horns, which are like a ram's horns, or four pairs of small horns. The cerastes hides in the sand so that only its horns show; animals, thinking the horns are food, come close and are killed by the serpent.

Aelianus [170-230 CE] (On the Characteristics of Animals, Book 1, chapter 57): The Cerastes is a small creature; it is a snake, and above its brow it has two horns, and these horns are like those of the snail, though unlike the snail's they are not soft. Now these snakes are the enemies of all other Libyans, but towards the Psylli, as they are called, they are gently disposed, for the Psylli are insensible to their bites and have no difficulty in curing those who have fallen victims to this venomous creature. Their method is this: if one of that tribe arrive, whether summoned or by chance, before the whole body is inflamed, and if he then rinse his mouth with water and wash the bitten man's hands and give him the water from both to drink, then the victim recovers and thereafter is free from all infection. And there is a story current among the Libyans that, if one of the Psylli suspects his wife and hates her on the ground that she has committed adultery; and if moreover he suspects that the child born from her is a bastard and no true member of his tribe, he then puts it to a very severe test: he fills a chest with Cerastae and drops the baby among them, just as a goldsmith places gold in the fire, and puts the infant to the proof by thus exposing him. And immediately the snakes surge up in anger and threaten the child with their native poison. But directly the infant touches them, they wilt, and then the Libyan knows that he is the father of no bastard but of one sprung of his own race. - [Scholfield translation]

Isidore of Seville [7th century CE] (Etymologies, Book 12, 4:18): The cerastes is a snake with horns like a ram's on its head; from this it gets its name, fro the Greeks call horns kerata. It has four horns, which it displays as bait, and instantly kills the animals it attracts. It covers itself with sand, leaving exposed only the part with which it catches allured birds and animals. It is so flexible that it seems to have no spine.