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 "Demodog" by the kids.
Mews was a cat owned by the Henderson family, which lived with them in their house in Hawkins, Indiana. It was especially loved by Claudia Henderson. Sadly, it was killed and eaten by D'Artagnan, a creature Dustin Henderson had adopted after finding it in the trash can. It was replaced by another cat, Tews at the end of Season 2.