Nathan F is absolutely correct. This is called a Dart. It was most famously known to be used my the Irish Kern when teamed up with a Gallowglass soldier (a Scottish mercinary) against a mounted knight. Gallowglas means "foriegn solder" . The Kern were experts with these darts in battle tactics while on foot with little or no armour. The idea was for the Kern to single out and harass a mounted knight and then to retreat thereby inviting pursuit by the knight toward a waiting Gallowglass. Much to the surprise of the knight, the more heavily armoured and outfitted Gallowglass would not run. The Gallowglass were known for being very good at pulling the knights off their horseback just before finishing them off.
I bought three Arms & Armour javilens a few years ago which were about the same length as the darts that are deplicted in the period art. I immediately noticed how difficult it was to throw them straight. If I didn't throw them perfectly, they would fish tail from side to side and sometimes hit the target sideways. Using the period art as a guide, I added two leather vanes to the ends of the javilens. The vanes stabilized the fish tailing and in fact made for perfect throws every time even while on the move. I have pictures of my darts somewhere.
Here is a picture by the famous artist, Albrecht Duer deplicting the Gallowglass and the Kern. http://upload.wikimedia.org/wikipedia/commons/e/ed/Gallowglass_-_D%C3%BCrer.png