Fallout Shelter creatures

Creatures attacking the vault
Creatures will randomly attack the vault in the form of incidents. This can also happen after failing a rush. Mole rats and radroaches multiply into adjacent rooms if there are no targets in the infested room(s), or if none enter the room within a short time after they spawn. Mole rats' and radroaches' strength depends on the level of the room in which they spawn. An average dweller level inside that room may also be a factor.

Allowing the intruders to multiply will likely result in the vault's destruction. Radscorpions also appear in incidents. Unlike mole rats and radroaches, radscorpions don't multiply. Instead, they transverse to a random room if they are not killed quickly enough in the previous room. They repeat the process until killed or leave after an extended period of time.

Creature encounter while exploring
Creatures can also be encountered by vault dwellers while out exploring, but will not physically be seen in the game. Killing these creatures gives the exploring dweller experience. If the dweller is too weak for killing the creature and must escape, they still gain ten percent of the score. The further the dweller(s) go into the wasteland, the more dangerous the creatures become.

Creature encounter during quests
Creatures encountered in quests can be seen, unlike creatures encountered while exploring the wasteland. Dwellers are able to inflict critical damage against creatures found during quests. When a creature is of a higher level than normal, a skull will appear next to their health bar.

Mammals

 * Feral ghoul
 * Glowing one
 * Mole rat
 * Mole rat brood mother
 * Super mutant

Creature encounters while exploring:

Reptiles

 * Deathclaw
 * Alpha deathclaw

Creature encounters while exploring:

Arthropods

 * Radroach
 * Glowing radroach
 * Radscorpion
 * Glowing radscorpion

Creature encounters while exploring: