This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Seps (Medieval Bestiaries)
A seps is a legendary snake from medieval bestiaries. They were said to have extremely corrosive venom that liquefied their prey. The seps is described as "a small snake which consumes with its poison not just the body but the bones" in the medieval Aberdeen Bestiary. Lucan's Pharsalia refers to its appearance and the effects of its poison.
A seps is about a meter long, and varicolored along its length; some say it can also change color like a chameleon. It uniquely has four hollow fangs in its lower jaw. Topsell attests to its speed, describing its motion as going “by spires and half-hoops”, possibly a reference to sidewinding.