Hydra
Ὕδρα
A swamp serpent that grew two heads for every one cut off.
It lived in the swamp of Lerna and was the target of Heracles' second labor.
He only managed to kill it when his nephew Iolaus seared each stump with a torch so no new head could sprout.
A "hydra-headed problem" is one where solving a single issue spawns two more.
SourcesHesiod, Theogony