Moai Dimension is two (or four on higher loops) Moai heads situated inside a rotating room. Their only appearance is in Gradius Gaiden.
Moai Dimension rotates around you, firing large rings that split into smaller rings, which in turn split as well. Once they're done rotating, one will be behind you, one in front. They will both fire a stream of heat-seeking rings. If you rotate around them, so that they go in opposite directions, you won't die. After a few more rotations, they will situate themselves above and below you, and continue their attack pattern as normal. Later, they'll return to normal. After one is destroyed (or three, on loop two and higher), the last one fires the split rings much faster. After they're all dead, the walls crumble and fly into the black hole in the background, bringing the Moai with them.