This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Scylla (Greek)
In Greek mythology, Scylla is a legendary monster who lives on one side of a narrow channel of water, opposite her counterpart Charybdis. The two sides of the strait are within an arrow's range of each other—so close that sailors attempting to avoid Charybdis would pass dangerously close to Scylla and vice versa.
Scylla is first attested in Homer's Odyssey, where Odysseus and his crew encounter her and Charybdis on their travels. Later myth provides an origin story as a beautiful nymph who gets turned into a monster. Scylla dwells with the Strait of Messina between Calabria, a region of Southern Italy, and Sicily.