The Eerie Smile is a hallucination in Mother 3.
It is not an actual enemy in-game, but rather how Lucas's party sees the enemies on Tanetane Island in battle after eating some "healing mushrooms", which cause considerable psychotropic effects. The abilities of the Eerie Smile vary, as they are all hallucinations of different enemies: the Monkalrus, the Ancient Dragonfly, the Titanian, and, in one incredibly powerful incarnation, the Zombieshroom.
As Lucas and his party fight an Eerie Smile, the smile's corporeal form will dissapate after a few turns, allowing the party to see the proper enemy they are fighting. Such enemies will not be recorded in the Battle Memory; thus, the only way to record the Zombieshroom is to attack it before eating the mushroom, which inevitably results in defeat for the party.
While fighting an Eerie Smile, the attacks they do will be narrated differently compared to the actual enemy's attack; for example, when a Smiled Monkalrus attacks using fleas, it will instead appear as "The Eerie Smile rocked from side to side."