One of the great things about season two of Stranger Things [1] is the exploration of Lucas's and Dustin's home lives. Sure, we know lots about Mike's and Will's families, but now we know a bit about the other boys' as well. We were introduced to characters like Erica [2], Lucas's sister who is the living embodiment of the eye-roll emoji toward him. We also meet Dustin's lovable mother [3] and his pets, Mews the cat and Yertle the turtle. He adds on another pet when he hears something in his trash can — and that something is what he believes to be a terrestrial pollywog [4] that he names D'Artagnan, or Dart. Before the reveal of Dart's true form [5], there's initially a lot of mystery surrounding what he is, but one question still remains: how did he even get into Dustin's trash can?
One possibility is that Will brings Dart into the real world. Remember when he coughs up a slug-like creature at the end of season one [6]? When Dustin shows off Dart to the rest of the gang, Will has a flashback to that moment. Could it be that the slug, who escapes through a sink drain, is Dart in an earlier form? We see Dart shed his skin and grow a few times in season two, and the slug is similarly slimy and clearly from the Upside Down. Dustin doesn't find his trash-can pet until almost a year after the creature would have left Will, though, so Dart would've had to lay low and survive for a long time. (Then again, Will would have been coughing up slugs on the regular.) He also may have many different forms before even getting to the misleadingly cute pollywog look, given how quickly he goes from that to full Demodog.
Another possibility is that Dart slips in more recently, perhaps through the Hawkins Lab gate. However, it seems they're pretty on top of burning things that come through, so he may not have had much luck getting into Hawkins undetected. Or perhaps he slips in through the rotting farms, since no one really knows what's up with them or why all the pumpkins are decaying for some time. Because the shadow monster's tunnels [7] are located underneath them, maybe Dart manages to easily escape above ground.
No matter the method of arriving in Hawkins, Dart finds himself in Dustin's trash can. He's presumably foraging for food — but he's clearly not a picky eater, considering he happily chows down on both nougat from Three Musketeers bars and Dustin's cat (#JusticeForMews). And while Dustin's adoration for Dart is initially super cute, it seems like common sense not to mess with unknown creatures. Didn't he watch the 1984 classic Gremlins?