"Recently discovered on Tanetane Island. It's usually rotten, and attracts its food by dancing."
Battle Memory
"Oh, Lucas. What are you doing here? Are you lost?"
— Zombieshroom, appearing as Flint after the party eats the Hallucination Mushrooms

The Zombieshroom is a mini-boss found on Tanetane Island in Mother 3, and must be fought by Lucas and co. after using the Hallucination Mushrooms.

After the party washes up on the shore of Tanetane Island with no items and 1 HP with 0 PP each, their only options to continue are to eat the Hallucination Mushrooms or defeat the Zombieshroom in their way. However, this enemy has stats that make it unbeatable in their current condition, so Lucas and co. (with the exception of Boney, who is healed regardless) are forced to eat the mushrooms to be able to defeat the Zombieshroom, who appears as Flint in the overworld but in battle appears as an Eerie Smile.

The only way to obtain the Zombieshroom's entry in the Battle Memory is to attempt to face it before using the Hallucination Mushrooms. However, this results in the party's defeat and subsequently continuing from the last Save Frog they saved the game at.

Like most bosses in Mother 3, it also has an unused back sprite, though it is just a mirrored image of its front sprite. This might imply that the Zombieshroom has two faces, though it could just be a dummy sprite that was scrapped.