From the Super Mario Wiki
“Clobber me for good life!”
Magmuses are Buzzy Beetles with rocks for shells that appear in Super Mario RPG: Legend of the Seven Stars. They reside in Barrel Volcano and are found quite common there. Magmuses have extremely high defense, although are relatively weak in comparison to other enemies found throughout the world. They are spewed out by the tree-like Stumpet, possibly indicating a relationship between the two species.
During battle, Magmuses attack by headbonking Mario and his allies. They can also increase their defense even more by using the special ability Valor Up. Since they are strong against many kinds of attacks, Magmuses are tough opponents, but an Ice Bomb or Mallow's Snowy works well.
Magmuses are the stronger relative of the Magmites.
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