This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Sea Serpent (Worldwide)
A sea serpent or sea dragon is a type of dragon sea monster described in various mythologies, most notably Mesopotamian, Judaeo-Christian (Leviathan), Greek (Cetus, Echidna, Hydra, Scylla), and Norse (Jomungandr). The mythical a creature that spans many cultures and millennia, often appearing in various myths, legends, and stories.
While physical characteristics also vary, sea serpents are typically described as sea-dwelling and vast, boasting features that combine serpent or snake forms with other marine animal parts, like legs and fins. Such creatures were based not only on mythologies but also on occasional sightings of unexplained wildlife in the oceans. Sailors who saw these strange creatures thought they were both the cause and the representation of the sea's violent, unknowable depths.