In season 2 of "Stranger Things," Dustin gives the pollywog the name D'Artagnan.
Dustin discovers the small, slug-like creature in his trash can after Halloween and assumes it is a harmless pollywog. He brings it home, hides it in his turtle tank, and feeds it candy, including a Three Musketeers bar. The name D'Artagnan references the swordsman from Alexandre Dumas's novel "The Three Musketeers"; Dustin soon shortens it to Dart.
The discovery occurs in "Chapter Three: The Pollywog," the third episode of season 2. Dustin's friends initially distrust the creature, especially because Will recognizes similarities between Dart and the strange organisms connected to the Upside Down. Their suspicion proves justified when Dart grows rapidly and is revealed to be a young Demogorgon-like creature, commonly called a demodog.
A common mix-up is answering "Dart," but that is the nickname. D'Artagnan is the full name Dustin gives it.