The Crypt rat can be found under the Barrows tunnels and in the Chaos Tunnels. Despite their combat level, they are weaker than they appear unless they come in large amounts. Due to the disgusting appearance of the crypt rat, not even a cat will try to catch it.
They do not pose a large threat to most players, but with many of them together along with other monsters, they become an annoyance. Unlike most rats, they ARE aggressive, and can usually land hits, unlike regular rats.
They can be handy as they can be used to heal if the player completing the barrows activity has Guthan's armour. Crypt rats are only aggressive for 10 minutes, after which they gain tolerance.
These are widely seen as a great way to get kill count in Barrows, as they can be killed quickly and don't pose much of a threat to players.