Warning: light spoilers for the second season of Stranger Things below!
One of the most terrifying storylines of Stranger Things's third season stretches back to a small moment that happens at the end of season one. If you remember, Will hallucinates an Upside-Down version of his home bathroom and coughs up a slug-like creature that slithers down into the pipes of the sink. In the new installment, Dustin comes home from his night of trick-or-treating to discover a small creature banging around in the trash can outside his house. In episode three, when he shows the rest of the gang, Will realizes this must be the same creature that he hacks up after his Upside Down ordeal.
When Dustin first finds this little guy, he doesn't realize it's a creature from the Upside Down. The slimy skin and appearance lead him to believe it's some sort of amphibian. He goes to the library and checks out a bunch of books trying to identify it, and by the time he introduces it to his friends, he says he believes it's some kind of "pollywog." In fact, the third episode of season two is titled "The Pollywog" because of this logic thread. But, like, what exactly is a pollywog?!
No, it's not that water-type Pokémom Polywag, although that cartoonish lil' guy gets his name from the same place. The term "pollywog" is actually just a fancy way of saying tadpole, or the larval stage in the life-cycle of a frog or toad. Tadpoles are basically just big round blobs with tails coming off them, so it's easy how Dustin's new pet could be mistaken for one in its infancy. Once a tadpole grows legs, it's officially a frog.
As the season rolls on, it becomes clear that Dustin's friend is not a pollywog at all, not by a long shot. At least, I don't think pollywogs grow Demogorgon-like faces and expand to the size of large dogs. But hey, never say never. Season two of Stranger Things is streaming now on Netflix.