Giant Rat

See the skull? It's actually a pixie skull. These are normal sized rats.

"I'm the Giant Rat that makes all of the rules!"

– The Rats song, by jerma985

D&D

Ah, the Giant Rat (also called Dire Rat): that most humble of beginner's level monsters, the traditional beastie you face when you're a 1st level jobber and worried that kobolds and goblins are out of your league. As their name implies, these are humble household rats bloated to unusual sizes - typically around the size of a dog - without any special powers beyond an increase in biting and clawing power. Arguably popularized by Dungeons & Dragons The Princess Bride, they are favorites of beginner DMs because they offer simple, easy-to-use enemies that won't require a greatly complex battle plan to oppose, are weak enough to face off against level 1 PCs, and come with an incredibly usable basic plot hook. After all, if you're a humble tavern-keep whose basement has been overrun by rats the size of Great Danes, would you rather face them yourself or offer the rookie adventurers in your common room a couple of gold pieces to stick them for you?

That said, giant rats aren't always so humble. Nastier versions do abound, and can catch you off-guard if you're unprepared - even the addition of disease to their attacks can wreck a careless PC at low levels.

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  B3: Palace of the Silver Princess

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  2e

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  4e

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  5e

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  PF 1e

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  PF 2e

Pathfinder

Pathfinder, in addition to the typical giant rats above, has a canine-like breed of giant rats known as Goblin Dogs. You see, in Pathfinder lore, horses and dogs instinctively hate goblins, so Paizo needed alternative beasties to fill these roles in goblin culture. Dire Wolves and Worgs serve as steeds and pack animals, while goblin dogs serve as guard dogs and hunting companions.

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Warhammer Fantasy

In the Warhammer Fantasy universe, Skaven often fight alongside giant rats specially grown to massive stature by the treatments of Clan Moulder, and they are considered the oldest and most reliable of that clan's fleshcrafted monstrosities. Many of them are further mutated for extra lethality, exhibiting features such as extra limbs/tails/heads, sharp claws, or spiny backs. Especially creative Master Moulders may even add grafted maces to their tails or exposed ribs that can be used as stabbing weapons, among more horrific "improvements".

Various editions have suggested the existence of a giant rat subspecies called Wolf Rats, which are generally characterized as being faster, stronger and more aggressive than regular giant rats, and/or giant rats bred to the point they can serve as steeds for skaven clan-leaders.

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Warhammer 40k

Yu-Gi-Oh

In Yu-Gi-Oh!, the card Giant Rat is an old (predates the English release of the game) Earth monster that is part of a cycle of weak monsters that special summon a low-attack monster of the same attribute (earth, water ect.), including other copies themselves, from the deck when destroyed by battle. Many of the common types (zombie, dragon ect.) have also gotten a monster that functions similarly. This series is often known as the "recruiters". They used to be pretty useful for thinning the deck, keeping monsters out even when the opponent has a strong monster, summoning monsters with low stats but good effect, filling the graveyard and the rare strategy that wants to destroy a bunch of their own monsters by battle (since the summoned monsters can attack immediately if you keep suiciding monsters into an opponent's). As such various members have seen competitive use on-and off through-out the years and are easily recognized by anyone who played in the older eras, but non-combat removal is so common now and the normal summon so valuable they're rarely seen today.