Scylla, in Greek mythology, was once a beautiful nymph, but when Poseidon had an affair with Scylla, Amphitrite Halosydne put a potion in the pool where Scylla bathed that turned her into a six-headed monster with a fish tail and dog heads sprouting from her body.
As a monster she lived on one side of a narrow channel of water, opposite of her counterpart Charybdis. The two sides of the strait were within an arrow's range of each other — so close that sailors attempting to avoid Charybdis would pass too close to Scylla and vice versa.
Scylla made her first appearance in Homer's Odyssey, where Odysseus and his crew encounter her and Charybdis on their travels.
The strait where Scylla dwelled has been associated with the Strait of Messina between Italy and Sicily. The idiom "between Scylla and Charybdis" has come to mean being forced to choose between two equally dangerous situations.