I was lucky enough to have the opportunity to play with a Phantom Omni Haptic device recently (if you are unfamiliar with what this hardware interface does, just give it a google, it offers incredible potential for 3D modeling). I loaded the Celtic Skull file into Geomagics Claytools, and with the Phantom Omni 'pulled' horns out of the skull.

So... enjoy this slightly more demonic augmentation of our friendly Thingiverse Celtic Skull. Cheers!