Unpleasant Animal Counterpart

When the Beauty Equals Goodness, What Measure Is a Non-Cute? and Evil Twin tropes are applied to animals that are similar but not quite the same.

For example, mice are stereotyped as mischievous critters who are capable of being cute and lovable. Rats, due to being bigger and significantly less cuddly, are rarely given as much leeway. Calling someone "mousey" lacks a harsh cannotation, it just means they're quiet and easily startled; calling someone "ratty" or a "rat" implies they're dirty and disgusting. If the hero is a mouse, one of the villains or anti-heros will be a rat. One way to insult a mouse is to "mistake" him for a rat.

Some Examples of "Good" Animal vs "Bad" Animal:
 * Alligators vs Crocodiles
 * Ants vs Termites
 * Apes vs Monkeys (though depending on the story, either can be the good or bad one)
 * Bees vs Wasps
 * Butterflies vs Moths
 * Chimps vs Gorillas
 * Coyotes vs Jackals (though coyotes live in North America and jackals live in Africa and Western Asia)
 * Crickets vs Grasshoppers
 * Crickets vs Locusts
 * Dogs vs Wolves (though depending on the story, either can be the good or bad one)
 * Dolphins vs Sharks
 * Eagles vs Vultures
 * Ferrets vs Weasels
 * Foxes vs Coyotes
 * Frogs vs Toads
 * Grasshoppers vs Locusts
 * Horses vs Donkeys and Mules
 * Mice vs Rats
 * Mongooses vs Weasels
 * Monkeys vs Baboons
 * Octopi vs Squid
 * Otters vs Weasels
 * Pigs vs Boars
 * Pigs vs Warthogs
 * Rabbits vs Hares
 * Ravens vs Crows (though depending on the story, either can be the good or bad one)
 * Sheep vs Goats
 * Snails vs Slugs
 * Squirrels vs Rats
 * Swans vs Geese
 * Whales vs Sharks
 * Wolves vs Coyotes
 * Wolves vs Foxes (though depending on the story, either can be the good or bad one)
 * Certain breeds of dogs vs other breeds of dogs, such as Golden Retrievers vs Rottweilers or Pit Bulls.

However, a work need not have a direct face off to give off this vibe. Mistaking one for the other and having said animal correct you with distaste ("Oh heavens no, I'm not a toad, I'm a frog!") gives off the implication that one is somehow an inferior version of the other.

See I Am Not Weasel if being mistaken becomes a running gag (the mistake need not actually be considered offensive to the animal, just incredibly exasperating), Elves Versus Dwarves for a magical equivalent, Slobs Versus Snobs, which is often used to justify this Fantastic Racism, and Always Chaotic Evil if the animal's prejudices are justified. See also Nice Mice and You Dirty Rat. This trope usually follows Animal Stereotypes.

The "pleasant" and "unpleasant" animals can either be related, similar, or both.

Anime and Manga

 * Purposely invoked in Pokémon. The Chick of the season, May, catches a Wurmple. The Dark Chick, Jessie, also manages to catch a Wurmple. Now Wurmples can either evolve into a sweet looking little butterfly (Bug/Flying) or an obnoxiously colorful moth (Bug/Poison). Guess which girl gets which? This is a subversion though, as both Pokemon are actually quite bad, arguably the former being the worst. Dustox make messes when they eat, leaving trees barren whenever they swarm into an area and scatter noxious dust as a defense mechanism. Beautifly are actually quite savage despite their appearance, being very aggressive when provoked or disturbed and attacking by stabling with their proboscis.
 * Done all the time in the Pokemon games as well. Caterpies vs Weedles, Pidgeys vs Spearows, Ledyba vs Spniarak, and Skittys vs Glameows.

Film

 * In the first Stuart Little movie, a bully of George's gets punched in the face when he states "You're right, he's [Stuart] not a mouse, he's a RAT."
 * When Kermit the Frog is called a toad, he explains that "frogs are handsome, debonair and charming, while toads are ugly and give you warts."
 * While all the good characters in The Great Mouse Detective are mice, the Big Bad is a rat. Just don't call him that to his face.
 * Inverted in The Lion King - Pumbaa is a Warthog, and being called a 'pig' is his Berserk Button. Normally, you'd consider a warthog to be the Unpleasant Animal Counterpart to a pig.
 * In Inglourious Basterds, Hans Landa muses on the differences and similarities between rats and squirrels.
 * In the 2001 Planet of the Apes one gorilla was very offended when the Mark Wahlberg called him a monkey.
 * In the Shrek movies Donkey wants very much to be a horse.
 * In Finding Nemo, sharks can't stand dolphins because, according to them, they're show-offs (and liked by humans).
 * In Shark Tale, dolphins appear to be one of the few animals that sharks don't mess with (basically their equals). When a "vegetarian" Shark decides to live amongst fish, he disguises himself as a dolphin (even though dolphins also eat fish...).
 * Averted in Ratatouille where rats get to be the heroes for once (though no mice appear), though considering the title is a play on words, it's rather necessary.

Literature

 * In The Bible goats represent evil people, whereas sheep represent the opposite.
 * In the Redwall series, mice are good whilst rats are evil, and otters are good while stoats, ferrrets, weasels and martens are evil.
 * In Kim Newman's Professor Moriarty stories, the Professor breeds wasps, this apparently being the evil equivilent of Sherlock Holmes becoming a beekeeper.
 * In Garry Kilworth's House Of Tribes, the protagonists are all mice, while one of the antagonists is a mean dirty old murderous rat.
 * In Twenty Thousand Leagues Under the Sea, whales vs cachalots. Nemo is adamant against killing whales for sport, but has no problem with killing cachalots -- described as "cruel, mischievous creatures".