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 in 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 is immune to Thunder, Fire and Ice, and he is immune to all status ailments. Mario's Jump attacks are very effective, however, as Hidon is weak to them.
Hidon's first action in battle is to release four Goombettes that were trapped inside the chest ("Monsters are inside!"). Goombettes provide extra help to Hidon, biting the party members and sometimes poisoning them. Fortunately, Goombettes are weak against Thunder, so Mallow's Thunderbolt can greatly help in such a case, as it may defeat all Goombettes in one blow. If Hidon is alone, he will likely summon another set.
If Hidon is targeted with a normal attack, he will either attack in return, or use Carni-Kiss, which allows him to bite a party member for 2x the damage of his normal attack. The odds of each is 2/3 in favor of his normal attack, and 1/3 in favor of Carni-Kiss. Otherwise, Hidon will randomly cast Static E!, Willy Wisp, Flame Stone; Static E! inflicts Thunder damage to the party, while Willy Wisp and Flame Stone only target one party member. Of Hidon's single-target spells, the latter is fire elemental, while the former has no alignment and as such cannot really be protected against.
Defeating Hidon is ultimately worth the effort, granting 50 experience points and no less than 100 coins, and the chest he was hiding in also gives the very useful Safety Badge. When equipped on a party member, this accessory increases physical and magical defense by 5, and prevents all status ailments (contrary to its description, which states it only protects against Mute and Poison).