This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Jasconius (Medieval folklore)
A similar monster appears in the Legend of Saint Brendan, where it was called Jasconius. Because of its size, Brendan and his fellow voyagers mistake it for an island and land to make camp. They celebrate Easter on the sleeping giant's back, but awaken it when they light their campfire. They race to their ship, and Brendan explains that the moving island is really Jasconius, who labors unsuccessfully to put his tail in its mouth.
Most easily apparent about the Jasconius is its immense size. It is said to appear like an island. In the middle high german version of the Navigatio sancti brendani abbatis transcribed by Dr. Thomas and F. Shannon it is said to have a small forest, a beach and even a small river on its back.