From the Super Mario Wiki
Hidon is a green Piranha Plant with a red stalk, disguised as a green Treasure Chest that appears in Super Mario RPG: Legend of the Seven Stars. There is only one in the game, found in a room of the Sunken Ship, just before the flooded area of the ship. When closed, Hidon is totally similar to a regular Treasure Box; as such Hidon can easily prey on unsuspecting travellers searching for a treasure. If Mario hits Hidon from below, the chest ghost will reveal his true form and will engage a fight against him. Like his lookalikes Pandorite, Chester and Box Boy, Hidon is a powerful opponent, having both high attack and defense, as well as high HP. Even Mallow's magic cannot pierce his defense, as Hidon tends to resist Thunder, Fire and Ice. Also, status ailments are ineffective against this enemy. Only Mario's Jump attacks are really effective. Hidon masters several techniques, such as the dangerous Carni-Kiss that allows Hidon to bite a party member, causing severe damage. Hidon may also resort to Static E!, an electric attack that damages all the party members, Willy Wisp and Flame Stone, that causes significant damage to the target.
During the battle, Hidon will also release four Goombettes that were trapped inside the chest, as the message "Monsters are inside!" proves it. Goombettes provide extra help to Hidon, biting the party members and sometimes poisoning them. Fortunately, Goombettes are weak against Thunder; therefore, Mallow's Thunderbolt can greatly help in such a case, as it may defeat all Goombettes in one blow.
Luckily, Hidon holds a large amount of treasures. After Mario and his party defeated Hidon, they received 50 experience points, no less than 100 coins and drops the very useful Safety Badge. When equipped on a party member, this item increases physical and magical defense by 5 and prevents Mute and Poison attacks.