It's said that these insectoid beasts start off as a larval form that gets into a human's stomach via food. It grows rapidly, eating the person from the inside, and taking the victim's skull as its own head as it devours everything upwards. This is, of course, nonsense.In-game description
The Skullbug is the Stage 2 Magic form of the Rotten group. It has fairly balanced stats for an Undead beast, but its defenses and magic attack are higher than its other stats.
Skullbugs cannot be found in the wild. They are not used by any enemy signers, except for those participating in the Random Tournament.
A Skullbug can be evolved from an Oskapo. In the alpha version it cannot evolve again.