Various cartoons and toy lines, specially in the late 1980s and early 1990s, used repulsive slimy creatures and ooze to appeal kids, especially
boys (hence the trope name pun). Keywords were "gross","slime" (or if you're British "gunge") and "ooze".
Toys often focused on some gimmick based around some goopy liquid, and most of these franchises had an obsession with the toxic waste theme. That meant lots of villains, and sometimes heroes, based on mutant abilities, bizarre chimeras, and distorted bodies.
The theme gradually went away when more conventional comic book heroes started getting their own cartoon adaptations.
- Green slime was a Running Gag on kids' series You Can't Do That on Television.
- Ooze in general became an emblem of Nickelodeon, and the "main" form for their logo was a splat.
- The Real Ghostbusters got that directly from the movies. Its toy line sold all sorts of ugly ghost creatures that looked closer to extreme mutants than spirits. It also included synthetic "ectoplasm".
- The formula was copy-pasted for the Teenage Mutant Ninja Turtles, which included "ooze" and featured humanoid creatures covered with garbage and insects.
- Similarly Swamp Thing; both were kid-washed animated versions of violent comics. The latter succeeded only as a toyline, since only five episodes were made. Toxic Crusaders, of The Toxic Avenger fame, got a slightly better deal with 13 episodes.
- Mighty Max was created as a toyline specifically to cash on that trend, although its animated version was toned down with a graphic style closer to Batman: The Animated Series.
- Creepy Crawlers was also spawned from a toyline, this time without the stylish look upgrade for the cartoon.
- Initially, Mighty Morphin' Power Rangers cashed that cow too, with many of their adversaries looking like Otto Mann's acid dreams (Rita Repulsa probably took lessons from Rob Zombie himself).
- The villain of the 90s movie is a Blob Monster named Ivan Ooze who invokes this trope in-universe, selling children slime that brainwashes their parents.
- While the decision to adopt the show in the 90s probably came from this, the original Japanese Super Sentai series has a long history of really bizarre, disgusting monster designs. While the first few shows mostly had thugs in ridiculous costumes, Denshi Sentai Denziman's Vader Monsters entire gimmick was that they came from a planet where people had entirely different ideas of beauty and ugliness and were made by the designers to be as grotesque as a TV budget would allow. The franchise has never really looked back since.
- Goosebumps is not far from that.
- Tales from the Crypt and its animated series were kind of the sophisticated version of that.
- The Mutant League games are a nice example; they even spawned a cartoon that lasted more than one season.
- The Masters of the Universe villains, the Evil Horde, got an entire playset based around this concept, the Horde Slime Pit. A claw would reach up to ensnare an action hero, and then a dinosaur skull would open its jaw to drop the official "slime," a green semisolid that came in a canister, similar to the Ghostbusters product above.
- Mattel's line of "Mad Scientist" toys, which included a "Dissect-An-Alien" kit (complete with phosphorescent slime for "Alien Blood') and a "Monster Maker" kit that featured an acid bath that would dissolve the "monster flesh" from their bones.