Big Galoombas are enemies from the Mario series. They only appear in Super Mario 3D World in a couple of levels. As you can already tell, these Galoombas are much larger. They require two hits to defeat -- the first to stun, the second to kick away. Even though Galoombas existed in Super Mario World, Big Galoombas did not.
Super Mario 3D World
These enemies are exceptionally rare, appearing in only two levels. The first is Big Galoomba Blockade, where three Big Galoombas must be defeated in order to gain the Green Star and finish the level. The second is Gargantuan Grotto, where they appear alongside many regular enemies.