From the Super Mario Wiki
Amanitas are mobile fungus enemies found in Super Mario RPG: Legend of the Seven Stars. They live in the Forest Maze and disguise themselves as common curative Mushrooms (which are plentiful in the forest). When an unsuspecting mushroom gatherer approaches a concealed Amanita, the fungus creature will reveal itself and attack. However, not all Amanitas disguise themselves; some move freely on the ground. One Amanita even hides in an underground bush and ambushes travelers that pass by.
In battle, an Amanita attacks by twirling into one of Mario's party members. Aside from its regular attack, the Amanita can use the special attack known as Sporocyst to turn others into fungi. As a Mushroom, a party member cannot attack, use items, defend, or even move. The only positive side effect of being turned into a Mushroom is that a party member's HP will slowly recover. Another Amanita appears in Monstro Town acting as a lamp for the inn and has a connection with the Three Musty Fears telling Mario they'll be back. Since Amanitas are fungi, they may be distant relatives of Toads and Goombas.
Amanita is a real genus of fungi, many members of which look similar to the Super Mario RPG enemy, and one of which was the inspiration for the Super Mushroom in the original Super Mario Bros. While many Amanita mushrooms are toxic, some are commonly eaten in some cultures, and misidentifying the mushrooms can result in poisoning; the Super Mario RPG Amanita's behavior of disguising itself as a helpful mushroom is probably a reference to this. Unlike common Mushrooms, or other Mushroom species, Amanitas have their caps opened up like mature mushrooms in the real world.
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