Some of these monsters, like the Moss Huge encountered by the Hestia Familia expedition party for example, change appearance as they get stronger.

There are also certain monsters that have grown stronger than normal due to eating magic stones which are known as strengthened species (強化種). In order to protect themselves, in addition to becoming stronger, they eat the magic stones of monsters they defeat. Normal monsters view them as enemies instead of allies and attack them. These Xenos seem to be reincarnations of previous monsters that have been killed multiple times by adventurers. Unlike other monsters, they are intelligent, though some are unable to talk at all. Some monsters, through unknown means, have been reborn into an existence known as the Xenos ( 異端児 ( ゼノス )). Monsters until around the Lower Floors have low intelligence and often repeat assaults on instinct, but the ones in the Deep Floors have higher intelligence that they know the concept of retreating.

Monsters appeared from the dungeon even before the Gods and Goddesses came from Heaven and some still live on the surface in various places however, monsters on the surface use the power of their magic stone to multiply, and so those on the surface are far weaker than the same ones inside the Dungeon. They are the enemies of the people and will attack without any warning. Monsters are born from the walls and ceilings of the dungeon and fight adventurers. Monsters (モンスター) are creatures born from the Dungeon that serve as an enemy to the world.

Nidhogg creature