In my Monster Snobbery post I mentioned a creature that is unique to the Forgotten Realms setting. This is the Moray Rat, a vicious variation of the normal rat that is the staple monster killing diet of many low-level characters.
What makes the Moray unusual is that it has nasty backward angled teeth that mean that once it has latched on to its prey it cannot let go or be shaken off. It will hang on to its prey until it eventually bleeds out.
Game mechanics-wise treat the rats as standard rats but if the rat delivers a critical that does at least 1 point of bleeding damage per round then the rat is attached. It will no longer attack but it just thrashs around worrying at the wound. I would give the victim or helper +50OB to attack an attached rat but I leave it up to the GM to decide what should happen if you try to hit a rat attached to your own leg and miss.
There is of course the option of the Moray Giant Rat but that is your own choice.
Morays tend to be found in much smaller numbers than typical rats due to the fact that their natural internal squabbles tend to lead to more fatalities which keeps numbers down. They are perfectly suited to living in burrows, pipes and crevices where prey may wander in as opposed to living in large packs and scavenging.