D'Artagnan, commonly referred to as Dart, was a creature originating from the Upside Down that Dustin Henderson temporarily kept as a pet. Dart is from the same species as the original Demogorgon, albeit in an adolescent form, referred to as a "Demo-dog" by Dustin.
Stage Four: Dart eventually developed into a much larger creature, becoming roughly the size of a small dog. Dart's limbs lengthened, while his skin continued to darken. Notably, his mouth began to open like a flower, revealing rows upon rows of sharp teeth.Dart grew tired of nougat at this stage, becoming attracted to meat. Dart broke out of the tank and fed upon Mews, Dustin's unlucky pet cat.