Polukranos

Source: Mythic Odysseys of Theros p. 231

Polukranos, called the World Eater, is the mortal manifestation of the eternal ideal of a hydra. In its first incarnation, it fell from Nyx to the mortal world, and so great were its size and strength that the gods Nylea and Heliod combined their power to bind the monster deep within the Nessian Wood. That incarnation was slain, but the eternal ideal remains. A new incarnation of the World Eater is a fearsome omen, as the monster only appears when the pillars of the world tremble and terrible things are afoot in the realm of the gods. Its rampages often presage an age of menace for all civilization.

What krakens are to the sea and dragons are to the sky, hydras are to the lands of Theros. Various hydras dwell at the fringes of civilization, from the bog-dwelling hydras known across the multiverse to massive ironscale hydras that lurk in deep wildernesses. Beyond even these exist serpentine horrors born of the whims of foul gods, like the legendary hydra Polukranos.