The mad doc has been circuit-bending animals. You walk in to mop the lab, and he is putting more or less living white rats into an incineration box. You talk him out of burning the last one alive by saying that you will keep it as a pet, and never allow the outside world to see it.
Now you have a fucked up rat as a pet. It is three times the normal size, but its legs are short, weak, and stumpy. It is blind and it makes strange, low-pitched throbbing bleats when hungry. But you love your lab mutant rat, and you care for it every day. It likes you too.