This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Jaculus (Medieval Bestiaries)
The jaculus (or iaculus, meaning "thrown" in Latin) is a small mythical serpent or dragon. It can be shown with wings and sometimes has front legs. It is also sometimes known as the javelin snake. It was said that the jaculus hid in the trees and sprang out at its victims.
This is similar to Malagasy folklore identified with Ithycyphus miniatus, a small snake with v-shaped markings on its head resembling a speartip.
The force of it launching itself at the victim led to the association with javelins. Pliny described it as follows: "The jaculus darts from the branches of trees; and it is not only to our feet that the serpent is formidable, for these fly through the air even, just as though they were hurled from an engine." A jaculus is also found in the Old Norse romance Yngvars saga. Here it is actually said to be an extremely large dragon