Peter, James and Sirius were all Animagi. James turned into a stag, "Prongs", Sirius a dog, "Padfoot" an Peter the rat, Wormtail". An Animagus is a wizard or witch capable of turning into a particular animal and back at will.

Animagi can assume only one form. Furthermore, the animal form is not chosen by the user, but instead reflects one's inner nature. While in animal form, the witch or wizard have the abilities of the animal they have transformed into, while retaining the mentality of a human. Being an Amigus is difficult and there are only 7 registered. Which makes me think it is not a common skill.

I don't believe Peter was unable to change into a man he just didn't want to. I don't think Peter regretted the fact that he betrayed the Potters or framed Sirius for the murder of 12 muggles and sent him to Azkaban for 12 years. Peter sold out two of his best friends without a thought because he thought he would be handsomly rewarded by Voldie. If he regretted his actions, I believe his magical ability could have been effected like Tonks was... but he only cares for his own well being.

A rat seems to fit Peter perfectly... A rat is a person who abandons or betrays his or her party or associates, especially in a time of trouble and/or an informer.