Episode 6: "The Pollywog"
One of the most evocative titles of Season 1 was "The Flea and the Acrobat," in which the boys learned about interdimensional travel from their science teacher, Mr. Clarke. Pollywogs refer to the larval stage for frogs and toads, so it's possible that this episode will also contain some sort of scientific lesson, potentially about the slug creature that Will coughed up at the end of the episode. Really, the possibilities are endless: the pollywog could refer to the Demogorgon, Eleven, or any of the boys themselves.
There's also a military connotation to the term "pollywog" that could end up being important. In the US Navy and many naval organizations, "pollywog" is used as a term to describe any sailor who has yet to pass the Equator. After a "crossing the line" ceremony, the sailors are referred to as shellbacks. An article on the Navy's website says that the ceremony would include "a series of initiation rites involving harrowing and often embarrassing tasks, gags, obstacles, physical hardships, and generally good-humored mischief."