What or who was Scylla in odyssey?

In Homer's "Odyssey," Scylla is a monstrous sea creature that dwells in a narrow strait opposite the whirlpool Charybdis. She is depicted as having twelve dog-like heads and six rows of teeth, capable of snatching sailors from passing ships. Odysseus encounters Scylla while navigating these treacherous waters, and he must choose between facing her and risking the loss of his men or confronting Charybdis, which poses a different kind of danger. Ultimately, he sacrifices some of his crew to Scylla to save the rest.