Why Peter Pettigrew cannot be a Hufflepuff:
Hufflepuff are extremely loyal, and brave. This little rat is not.
Why Peter Pettigrew cannot be a Ravenclaw:
When it comes to brains, Peter is seriously lacking.
Why Peter Pettigrew cannot be a Slytherin:
Brains go hand in hand with cunning. You must have one to have the other. The rat does not.
Why Peter Pettigrew, the rat that he is, has to be in Gryffindor:
Daring, nerve, and chivalry. Although he most likely had very little chivalry, he showed a tiny bit of nerve and daring when he hesitated in killing Harry Potter. And he became an illegal Animagus. Yes, he probably did that out of fear of loosing his friends, but still.
This is why Peter Pettigrew can only fit in slightly into Gryffindor.
A/N: Well, it's been annoying me for a while. I've read fics that put Peter in Hufflepuff. Hufflepuff! Hufflepuff are, above all, loyal. They are quite willing to lay down there lives for a friend. Can you see my reasoning? And then I thought, Slytherin? No, because Peter has enough pride to fill a thimble.