Basilisk

The basilisk held dominion over all serpents by birthright. Hatched from a cock's egg incubated by a serpent — or so later traditions claimed — the original Greek basilisk was a small snake from the Libyan desert, no longer than twelve fingers. What it lacked in size it made up for in lethality. Its breath scorched grass, split rocks, and killed any creature unfortunate enough to inhale it.

Pliny recorded that the basilisk moved with its body raised upright, the front half lifted off the ground, a crown-shaped marking on its head giving it the name "little king." Other snakes fled at its hiss alone.

Only two things could kill it. The weasel, immune to its venom, could be thrown into the basilisk's hole to hunt it down — though the weasel typically died too. And the rooster's crow sent the creature into fatal convulsions, a detail that made roosters prized travelling companions in serpent country.

Alexander the Great's army reportedly lost soldiers to a basilisk in the Libyan wastes. The creature had nested near a well, and men who drank the water died within hours. A scout identified the tracks and the well was sealed with stones.

Medieval bestiaries inflated the basilisk into something far larger and more fantastical, but the Greek original remained a small, terrifyingly potent snake — proof that in mythology, the deadliest things often come in modest packages.