Basic Trope: RPG uses rats as the first encountered, lowest level enemy.
Straight: Alice enters an AdventureTown hoping to become a mighty warrior and a hero, but before anyone hires her for any quest, she must prove her worth by killing some rats.
Exaggerated: Quite a number of the first quests don't have any other enemies besides rats.
Downplayed: The first enemy Alice encounters in the tutorial dungeon is a rat, but it's the only one.
The townsfolk are used to would-be-heroes who in fact aren't even capable of killing a rat. To avoid sending completely inept people to their deaths on real missions, they use a basic rat-killing quest as a litmus test.
Or, Alice enters a dungeon via sewers, but being such a wimp, even the rats that live in the uppermost levels pose a real threat to her.
Alice's My First Adventure consists of her protecting some rats.
The main character is a rat and has to fight people at some point.
Subverted: The rats Alice encounters during her first quest are actually formidable enemies, and she's supposed to avoid them.
Double Subverted: The rats Alice encounters during her first quest look an awful lot like quick and painful death, but it turns out they actually are low-level and easy to kill.
Parodied: Rats are level 0, and completely harmless. If Alice kills one, the game taunts the player for being such a wuss.
One of the first available quests for a low-level character is indeed about killing rats in a basement, but the rats are very high level enemies and the quest cannot be completed (or even survived) until a lot later. However, the townsfolk treat Alice like a child and refuse to sell her any weapons or other equipment until the rat quest is finished, forcing her to make do with what she can obtain by herself.
Alice is looking for her first quest and finds a merchant who reluctantly mentions some rats in their cellar. She dashes off to kill the things only to have a horrified merchant shriek at her for slaying the pet rats she was supposed to herd back into their cages. Alice must find them three new Hershonian Blues somehow... But now she's got a bad reputation and she needs to work her way up to be even trusted with ratslaying jobs...
Averted: Rats are not used as enemies at all. Optionally, all or most of the enemies in the game are rats.
Enforced: "Guys, the boss man called and said we have to do rats for the first dungeon."
Lampshaded: "Well, young stranger, the rats seem to always grow aggressive when a new would-be-hero sets foot in town."
"Sheesh, I'm way too wimpy and inexperienced to be any kind of a hero. If only there was a weak enemy I could practice my sword-fighting on... something small and not too dangerous, like rats."
Alice was a struggling pest killer... then she started playing the role of an aspiring hero, and work's never been easier to find.
Defied: Even if rats exist in the universe, there is no way Alice can kill them.
Discussed: "Kill the rats in your basement? Well if that's what it takes to make you trust me with the real jobs."
Conversed: "You'd think even the devs would get tired of rats as the first battle encounter, but no."
Deconstructed: Despite being an incredibly easy to dispatch foe, the rats end up being a genuine danger to Alice's health because of the infections and parasites they have acquired after roaming around heavily polluted areas. Even a little scratch would be enough to contaminate a person with a potentially lethal illness.
Plotted A Good Waste: The rats Alice runs into the first thing were bullied by the other creatures to stay behind and buy them time to flee.