Needs a better title.
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.