Eerie is the ghost of a dinosaur-like creature which flies in its sleep. Eerie has a round Boo-like body with large scales running down its back and a head with a pointy snout and closed eyes. They reside in Dinosaur Land's ghost houses and fly either in a straight line or a wave motion. Sometimes, five of them will fly in formation in a wave motion, which is particularly hard to avoid. There is also an Eerie generator which will cause Eeries to fly in from both sides of the screen at random heights. This generator was never used in the original game.
Mario will take damage if he attempts to stomp an Eerie, but he can spin jump on them and can even "ride" them for long distances if he times it right. Eeries are immune to normal attacks such a fireballs, cape swipes, and thrown objects, and Yoshi cannot eat them. Stars can defeat them, as can Bob-omb explosions.
A normal Eerie is sprite 38, a wave Eerie is sprite 39, a group of five Eeries is sprite DE, and the Eerie generator is generator CB. Eeries use sprite palette E, which is one of two palettes which can differ from level to level depending on which Sprite Palette the level uses (the Eeries use Sprite Palette 5). Eerie's tiles are spread out across SP3 and SP4, so it can only be used when SP3 is set to GFX06 and SP4 is set to 11.
The Eerie generator is often added to Big Boo boss battles to make it more difficult for Mario to beat him. The Eerie generator has a habit of sticking Eeries halfway into the floor, so that they must be jumped over but cannot be spin jumped, which considerably raises the difficulty.