Which six-headed sea monster lived opposite Charybdis?

The story behind the answer

Scylla is the six-headed sea monster who lurked in a cliffside cave opposite the whirlpool Charybdis, famously threatening Odysseus in Homer’s Odyssey. Sailing the narrow passage (often linked to the Strait of Messina), crews faced a grim choice: risk total destruction by Charybdis or lose six men to Scylla—giving rise to the idiom “between Scylla and Charybdis,” akin to “between a rock and a hard place.”

To remember: Scylla = six heads on the rock; Charybdis = churning maw below. By contrast, Cetus is the monster in the Perseus-Andromeda myth, Leviathan is from Near Eastern/Biblical tradition, and Triton is a merman sea god, not a monster.