The Flying Skull is a monster found in the Underworld, added by the N Terraria Mod. It takes on the appearance of Skeletron's (or perhaps more accurately, the Dungeon Guardian's) skull, but unlike most Underworld creatures, it can be burnt by the lava it lives by. 

Due to a bug in N Terraria's latest version (5.18), it appears as though two skulls are stacked on top of one another, when in fact the skull is just spinning in a strange way.