This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Isonade (Japanese)
The Isonade is an enormous, shark-like sea monster said to live off the coast of Matsuura and other places in Western Japan. According to the Ehon Hyaku Monogatari, its body has never been seen, as it is always "hidden beneath the waves, save for its huge tail fin which is covered in small barbs."
They use these barbs to hook their prey, dragging them deep into the water to be eaten. Isonade are said to appear when the north winds blow and the sea currents change. Despite their size, isonade are incredibly elusive. They move through the water with unparalleled grace and can swim without creating so much as a splash.