Here is a list of mythical creatures beginning with this letter and I am constantly gathering information from each continent and continually updating.
Ayakashi (Japanese)
Sea serpent that travels over boats in an arc while dripping oil found in the waters around Japan, the Ayakashi Ikuchi secrete an oily substance which they drip over boats as they arc out of the water. Known for being incredibly long, they will often follow a boat they are arcing over, making it very difficult for sailors to avoid the oily goop the creature secretes.
Most usually they frantically bail it over board manually or magically but some boats will actively seek out the creatures in order to collect the oil, as it can be useful in waterproofing potions. They are, thankfully, rarely seen, and rarely cause too much trouble for Japanese sailors. Those that have encountered the creatures have kept remarkably complete notes on them and their appearance, and, due to this, current consensus is that Ayakashi Ikuchi are some variant of Sea Serpent, likely one which prefers the deep sea to the surface.