D'Artagnan

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 Dustin.

1984
When returning home after trick-or-treating, Dustin heard a noise coming from the trashcan. Upon investigating, he discovered a creature and placed it in his ghost trap. Dustin placed the creature in his turtle's tank and fed it a Three Musketeers bar, appropriately giving it the name D'Artagnan after the character from the novel.

The next day, Dustin brought Dart to school, showing it to Mike, Will, Lucas, and Max. After discussing what type of animal Dart could be, Dustin concluded that he had discovered a new species. However, after noticing it resembled a slug he had thrown up the year before, Will realized Dart originated from the Upside Down. Before Dustin could show Mr. Clarke, Mike took Dart and urged everyone to meet again in the AV club room, excluding Max. When Dart started to emit a screeching sound, the boys opened the proton pack to witness it grow a pair of hind legs. Mike then tried to attack Dart, which caused it to flee and run loose throughout the school. Will eventually found Dart in a bathroom; it screeched, causing Will to run away. Soon after, Dustin arrived to retrieve Dart. When the others arrived, Dustin hid Dart under his hat and lied saying he had not seen it or Will.

Dustin then brought Dart home, and continued to care for it secretly while keeping up the appearance of looking for it with the others. After returning home from school, Dustin found that it had broken out of the tank and eaten the Hendersons' cat, Mews. Realizing Dart was now dangerous upon seeing how it could open its mouth like the Demogorgon, he lured it into the storm cellar and called in Steve to help exterminate it. However, Dart escaped, forcing Dustin, Steve, Lucas, and Max to lure it to the junkyard in a final bid to kill it. Dart soon arrived but was joined by several other creatures of its species, which started to attack the group until they were called to Hawkins Laboratory by the Mind Flayer.

Dart was not seen again until the group were trying to exit the tunnels underneath the pumpkin patch, when they were stopped by it blocking their way. Recognizing Dustin, Dart did not attack and was easily distracted by Dustin giving it another Three Musketeers bar, allowing the group to escape. Dart did not join the other Demo-dogs in returning to Hawkins Laboratory, and was ultimately killed when Eleven closed the Gate to the Upside Down.

Abilities
Being an adolescent Demogorgon, Dart possessed abilities that are characteristic of his species: Additionally, Dart had the ability to “block out” the Mind Flayer's influence; he was able to recognize Dustin when they reunited in the tunnels, and ignored the Mind Flayer's command of returning to the Gate. It remains to be seen if other Demogorgons are capable of this ability, or if it was somehow exclusive to Dart.
 * Strength: After molting to his Demodog form, Dart possessed enough strength to tackle an adult human.
 * Durability: As a Demodog, Dart's skin was thick enough to withstand several gunshots.
 * Digging: Dart could dig tunnels in the earth, allowing him to escape the Hendersons' storm cellar.

Trivia

 * The creature has various similarities to the Gremlins from the 1984 comedy horror film Gremlins. Much like Billy, the main protagonist, Dustin keeps Dart as a pet, initially seeing it as harmless until it evolves into a much more dangerous creature.
 * One of the species of frog that Dustin initially believed Dart to be is the Indirana semipalmata. However, the Indirana genus wasn't named until 1986.
 * D'Artagnan is named after one of the musketeers from 'The Three Musketeers ', because the first thing Dart ate was a 3 Musketeers chocolate bar.