These rats are found in the Deeprun Tram area.

When a player uses the taming flute, the normal rat is despawned and one of these is created. It then follows the player.

There is no limit to how many rats you can have following you. I like to create little armies of them and claim to be their prophet, leading them to the new world. It is not possible to leave the Deeprun Tram area, and they can't get on the tram with you. Also, they despawn after about ten minutes of following you.