Monster of the Aesop

"Buffy: I told one lie, I had one drink.

Giles: Yes, and you were very nearly devoured by a giant demon snake. The words "let that be a lesson" are a tad redundant at this juncture."

- Buffy the Vampire Slayer, "Reptile Boy"

The tendency of the Monster of the Week to conveniently fit to the episode's theme and Aesop in its appearance and mannerisms, even if they were supposed to be random.

This is frequently justified in one way or another. In some series, there's an inherent property of the Monsters of the Week that makes them tend to follow this trope, even tailoring their Sculpted Physique to fit that theme. Other times, the villains are actually inspired by or even taking advantage of the plot of the episode.

Often the monster's nature can be connected to a Double Aesop, either on learning its origin or finding the way to defeat it.

See also Mono Gender Monsters and Astonishingly Appropriate Appearance.

Anime and Manga

 * In various versions of Pretty Cure, the monster is formed from handy, usually inanimate objects, so if any one thing has been the focal point of the episode, there's a good chance the girls will end up fighting it at some point. Of course, the Monster of the Week is just as frequently something random, like a piano or a planter or a gazebo.
 * In the kiddie anime Zettai Muteki Raijin Oh, the monster seeds were specifically activated by the word "meiwaku" (troublesome, problem) being used in a phrase, and would then take on the form/powers of whatever was being considered a problem by the speaker. So there was a traffic jam monster, a flu monster, a superhero monster (this one had some serious "what side am I on again?" issues) and so forth.
 * In Sailor Moon, it often occurred either because the monster inherited the human host's traits in grotesquely exaggerated form, or because it possessed an inanimate object owned by the Victim of the Week and somewhat connected to the episode's plot. Most of the monsters in the first few episodes didn't follow this precisely, mainly being heavily bowdlerized versions of Cutey Honey type monsters.
 * At least one example wasn't backed up at all. There were twin demons, called Castor and Pollux, who maintained close relations and perfect conformity in actions and even had their tails knotted together. Of course they reiterated the importance of cooperation like every five seconds. They held rather well against the Sailors, but then all of a sudden started to argue about who's going to finish the beaten girls and in about two seconds their friendship was over, their link broken and they were summarily wiped out.
 * CLAMP has used this a few times in both Tokyo Babylon and XXXHOLiC.
 * All the X Eggs in Shugo Chara stem specifically from self-confidence issues.
 * Except in filler episodes when "? ("Mystery") Eggs" take on this role
 * Pixy Misa in Magical Project S would frequently create a "Love-Love Monster" from an object linked to the story (e.g. P.E. equipment, a chemistry vial, a comic book, etc.).
 * Neon Genesis Evangelion features this occasionally with the Angels, the most notable example being Israfel, which forces Shinji and Asuka to overcome their hostility towards each other to defeat it.
 * Leliel seems to exist for the sole purpose of waxing philosophical about Shinji's inner mind. We do not talk about Arael.
 * In a similar vein to the Raijin-Oh and Sailor Moon examples above, Gao Gai Gar's monsters of the week through the first half of the series were the manifestations of the stress that the host of the Zonder Metal was suffering from.

Live Action TV

 * Power Rangers. Rita Repulsa, in particular, did it all the time. In fact, most villainous plots from the first two seasons were inspired by what the Ranger teens were doing at school when the villains looked in.
 * Zedd was, in a way, worst, as he'd make monsters from (sometimes Aesop-related) objects, allowing them to be even better tailored to the plot!
 * To be fair, sometimes Rita and Zedd would use a Ranger's worst fear or squeamishness, such as Kimberly's revulsion towards frogs and Zack's fear of spiders.
 * In the Charmed episode "Battle of the Hexes," Billie is rather outspoken about her beliefs that Straw Feminism is better than real equality between the sexes. She learns the defects of this philosophy when she just happens to discover a magical belt that belonged to an ancient Amazon queen. More or less by coincidence, the demon of the week also holds to Straw Feminist views.
 * In the Buffy the Vampire Slayer episode quoted above, Buffy and Cordelia go to a frat party. The one drink is, of course, drugged, though this being Buffy the girls are nearly devoured by a giant snake monster as opposed to raped. The next time Buffy tries drinking with frat boys, they all turn into cavemen (and cavewoman) stereotypes. Like most Monster of the Week shows, the monsters are often Anvilicious.
 * Doctor Who uses this every other story.

Western Animation

 * Filmations Ghostbusters, to an extent. It doesn't quite work so well because Prime Evil continually relies on his "Usual Gang of Idiots" to do his bidding.
 * There actually aren't very many episodes that use Monster of the Aesop. There are many episodes that have AN Aesop, but that's not the same thing.
 * Each episode of Yogis Gang featured a villain who encouraged or reveled in bad behavior such as Greed, bigotry, vandalism, or littering.
 * A common theme on Sushi Pack is that the villain of the day will need (and usually fail) to learn the same lesson that one or more members of the Pack is struggling with.
 * Common on Teamo Supremo where the villain's M.O. would often coincide with some issue one of the heroes was struggling with, such as a battle with Sloppy Joe coinciding with Brenda having problems with disorganisation.
 * In the 2007 Teenage Mutant Ninja Turtles movie, the Turtles face a villain-led "brotherhood" while they themselves are having unity issues. Just prior to the final battle, the Turtles' family comes together as that of their enemy falls apart.
 * In the Teen Titans episode "Forces of Nature", the other Titans are mad at Beast Boy for his non-stop, often dangerous pranks, which he unapologetically commits. The villains of the episode are then revealed to be Thunder and Lightning, a pair of superpowered brothers who turn out to be less evil than... a pair of pranksters who don't fully grasp they're hurting people. They and Beast Boy then learn the aesop about taking responsibility for one's actions together.
 * To be fair, Beast Boy was aiming the major prank in that episode at Cyborg, and although it hit Starfire by accident, it's established that Cyborg and Beast Boy do this sort of thing all the time.