St. James Matamoros & St. George

[Rosina Lila]

The images of Santiago Matamoros and St. George slaying the dragon are very much alike: both on horseback brandishing a sword with defeated foe underfoot.  Which migh have been first? Is one a copy of the other?
In Leyre, the Cathedral of Pamplona,Rosina and countless other churches there are many depictions of Moors beheading Spanish women and children.  In Leyre there is some veneration for two sisters who were so killed.  Was beheading that common among the Moors? If so, why?
Thank you for whatever information you may share in this regard.