Here is a list of mythical creatures beginning with this letter and I am constantly gathering information from each continent and continually updating.
Aspidochelone (Medieval)
The aspidochelone is a fabled sea creature, variously described as a large whale or vast sea turtle, and a giant sea monster with huge spines on the ridge of its back. No matter what form it is, it is always described as being so huge that it is often mistaken for an island and appears to be rocky with crevices and valleys with trees and greenery and having sand dunes all over it.
The name aspidochelone appears to be a compound word combining Greek aspis (which means either "asp" or "shield"), and chelone, the turtle. It rises to the surface from the depths of the sea, and entices unwitting sailors with its island appearance to make landfall on its huge shell and then the whale is able to pull them under the ocean, ship and all the people, drowning them.