This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Sisiutl (American Indian)
The sisiutl is a legendary creature found in many cultures of the Indigenous peoples of the Pacific Northwest Coast, notably the Kwakwaka?wakw people group. Typically, it is depicted as a double-headed sea serpent. The sisuitl features prominently in Pacific Northwest art, dances and songs.
The sisuitl is closely associated with shamans because both are seen as mediators between the natural and supernatural worlds. Kwakwaka'wakw depictions often show a central, humanoid face between the double heads. All three heads are usually horned, and the serpent heads typically have protruding tongues and spiral or upturned noses, with crescents along the body.