A snake. May be blazoned a “viper,” a “snake,” or any period variety of snake. The dragon-like head shown here is typical of early depictions of serpents. This arrangement with a serpent azure or vert is restricted as a badge of the Visconti family of Milan, where it is known as the “biscione.”
http://heraldicart.org/serpent/
Adapted by Mathghamhain Ua Ruadháin from a mosaic of the arms of the Visconti family.