Some people or pet stores call natal rats dwarf rats when they aren't rats at all. Here in Montreal, there was half a dozen rats abandoned in boxes outside a suburban SPCA and two of the females were full grown but tiny.
This is Momma, with her two week litter:
The pictures I took do not do justice to how truly tiny she was. smallvic adopted her two sons and although bigger than she is, full grown they are tiny boys. If I had to guess, I would say Momma weighed in at a whopping 180g and her sons at 320g.
This is Bandit (capped girl) with the third girl abandoned that day Amy (agouti hooded) and some of my beast during intros:
She is also quite small (although nothing like Momma) and has weighed 340g since the day she arrived. By comparison Amy, whom she lives with, weighs 480g and the tiny girl in my large colony, Oreo, who is skinnier than Bandit weighs 390g. Here's a pic of Amy, Bandit, Oreo (black dumbo hooded), and Butterscotch's nose:
I should also note that, like all of my rats, Oreo is steadily gaining weight as she ages (5-10g per month) except Bandit who is stable.
To finish answering the question, there is no such thing as "dwarf" rats, although there can be some that are genetically programmed to be smaller. I do not know of any reputable breeder that purposely breeds rats to be "dwarf" or "giant". (I have seen "giant" rats for sale around here.) Needless to say, the suspected breeder that abandoned these rats was not reputable.
