Hmmm...the worst I had was a chin I got from a breeder that never gained weight. Her stool tests and blood were fine, no Giardia, nothing wrong. For about 5-6 months we hand fed this girl, and she never got above 300g. So at 8 months, she was under 300 grams. I even thought about putting her down, she just looked so sad and sick and in pain. But, one miraculous day, she started eating and is now 800 grams! Not a darn thing wrong with her. I honestly think it was that she did not like the food, but cannot prove it. She started eating right around when we got new food (from Tradition to PANR) and I will always suspect something. I would mix her critical care with crushed up pellets, and that is when she started eating again. Picky girl! She grew out to be a just gorgeous tan white girl.
I have had quite a few rescues come in very skinny- 380 was the worst I think. More come in fat though, bad fat from crappy food.