Rat-Maso

Rat-Maso is one of the new monsters in the Novasphere Pioneers expansion for the MMORPG Elder Tale. He is a boss of the Depths of Palm dungeon in the anime.

Overview
Rat-Maso has the form of a giant green-furred Rat-Man and it is a level 68 monster. It apparently has a reeking odor.

Saving Serara arc
During the travel to Susukino, in order of rescue Serara, Shiroe's Party enter in the Depths of Palm. There they realize that parts of the dungeon is cracking, something they note was not in the game.

When they arrive in the final zone Shiroe, Naotsugu and Akatsuki encounters Rat-Maso, concluding that it must be the reason the dungeon is falling apart. They decide to take the risk and destroy it clearing a way to the end of the dungeon.