The Muzzled Mouse is a starved rat wrapped tightly in leather to disable its limbs, with only its mouth (with extraordinarily sharp teeth) left uncovered. Its owner feeds it only occasionally and the rest of the time its mouth is clamped shut to prevent accidental biting.
The Muzzled Mouse is kept in a strong leather bag and handled with care (have you ever been bitten by a rat?). When needed it can gnaw through three inches of wood in its blind and vicious hunger, or it can be used as a lockpick.
They often need replacing, since the rat may die from starvation, splinters or bleeding. Foolish owners who are careless get bitten by their rats, and often contract unpleasant and incurable diseases.
Sometimes you can find these little forlorn bundles of leather-bound flesh lying in alleyways, having been discarded by thieves for their broken teeth.