“These pests are a nuisance and have a nasty habit of burrowing into your Vault from the dirt and chewing on any room's power cables. Kill them quickly, or they'll drain your Vault of Power!”— In-game description, under help.
Mole rats have sharp claws and even sharper tusks that they use to tear great chunks of flesh from their prey. Mole rats are either huge rats or the result of gene-splicing Kodiak bears with maladjusted lab rats. Fortunately, they seldom run in packs larger than a half dozen or so.[Non-game 1]
Mole rats can be encountered exploring the wasteland and during incidents. They are stronger than radroaches and weaker than deathclaws. (They would only spawn in rooms that form the vaults boundaries - Until late 2018), but if not dealt with quickly they can move further in to the vault.
During incidents, mole rats will quickly drain the power in the vault. They will start appearing once one reaches 22 dwellers in their vault (in hardcore mode). Like radroaches, when spreading to other rooms, they will crawl back into the hole they came out of and then into the target room.
They can also be encountered during quests. Their boss variant (mole rat brood mother) is tougher, stronger and larger in size and has the ability to call for reinforcements. This ability is not very often used and can be interrupted.