Terrifying Pet Store Rat

The protagonist opens the cupboard in a haunted house and, terror of terrors, there's a rat inside. Everyone involved screams at the sight of the filthy diseased vermin. Except it looks like it's been recently bathed and brushed. And instead of fleeing or acting aggressive, it's looking expectantly at the actors for a treat. In really egregious cases it might even be white or multicolored instead of brown like a wild rat.

This is a Terrifying Pet Store Rat. While the animal in question is often a rat, it also applies to spiders, snakes, and anything else the audience is expected to react with fear or revulsion toward, despite the animal reacting like a pet. Spiders will saunter calmly over people instead of racing along in a panic. Snakes will crawl onto people's shoulders, staring them in the face comically instead hissing and coiling defensively. In particularly bad cases "angry" dogs may have dubbed in growling while their tails wag.

This is a stock feature of horror movies, they often wander in the background for ambiance, act as a Cat Scare, or provide a Why Did It Have to Be Snakes? moment when a protagonist has to get past them. In severe cases, the central threat of a horror movie may fall into this trope. The main thing making this trope is that the animal is neither threatening nor believably wild.

Examples of Terrifying Pet Store Rat include:

Graveyard Shift, a movie based on a short story about killer rats by Stephen King, uses several scenes of rats sitting placidly along the rafters staring at the characters in a way reminiscent of The Birds. Fortunately the movie shifts the actual monster to a giant bat.
Disney's The Haunted Mansion movie at one point has the son need to get past spiders to get into the mausoleum. The spiders are harmless tarantulas (orange-kneed, I think) and they're tame enough that he can move them with his hands.
Hilariously done in the exploitation film The Gestapo's Last Orgy, where a woman is threatened by being hung over a pit of flesh eating rats... Played by gerbils.

Nazi: If I didn't take my hand out, they'd strip it down to the bones in a minute.
Cinema Snob: Yeah, sure. If your hands were made of windblown seeds and grain.

Played effectively in Willard, since the rats are supposed to be tame, well-groomed, and friendly because Willard takes care of them. It's just that they'll kill if they're told to.
The Indiana Jones franchise does this extensively. Especially in Temple of Doom.
Played for Laughs in Team America: World Police where some "panthers" are clearly house cats.
Played for Laughs in South Park when the town is overrun by giant guinea pigs and other "guinea" animals such as rabbits and dogs, the animals in question are simple shots of pet animals shuffling about benignly in cute outfits, which have been digitally inserted into the animation.
- Also spoofed with the Mexican Staring Frog of Southern Sri Lanka, which isn't even a real frog, just a stuffed toy on a string.
Bones uses this from time to time when a corpse is found infested with animals eating it.
- Frequently when they find a decaying body in the sewer covered in rats, none of which show the slightest surprise or interest in the living humans walking about.
- In one episode they find the victim surrounded by possums. Again, no possum shows any hint of non-familiarity with humans.
- In another episode the dead Victim of the Week is surrounded by "feral" stray cats, which sit about and let themselves be picked up by animal control.
There's a History Channel documentary on the Black Death that uses hooded rats in its scenes of flea-infested rodents carrying the Plague into port. Granted, they look a bit grubby, but their coat-pattern mutation is still conspicuous and unlikely to survive in the wild.
- Not to mention that the rats who contributed to the Black Death were black rats (Rattus rattus) not brown rats (Rattus norvegicus, the rat you're more likely to encounter on the streets or in a pet shop)
The English-speaking production of 1931's Dracula didn't even have the nerve to use real rats, but opossum and armadillo stand-ins.
Night of the Lepus. The infamous Giant Killer Bunny Rabbits movie. The rabbits look like a menagerie straight from a pet store (to be fair, there're only a few lops) which are mostly shown running through miniature sets in slow motion or in extreme close-up, smeared with red paint and backed by dubbed growling.
The Killer Shrews uses dogs in bad costumes to play the shrews. As you might guess from the quality of the effects, the dogs were not trained well enough to act scary either.
The Stuff uses a great dane which is apparently threatening its owner if it doesn't get more of the titular substance. It wags its tail throughout the scene while dubbed in growling plays.
Life After People, if not using Conspicuous CGI for the rats.