Giant Rat

A giant rat as ally is the pet rat the wizard found in the sewers and refused to leave behind. Run it as a familiar-adjacent companion that the table treats with affection, not as a combat asset. With 7 HP, no Multiattack, and no listed Bite damage in the stat block, the rat is not contributing to encounters. It is contributing to scenes.

The play is utility. The rat squeezes through the iron grate the party can't open, scouts the room beyond, and comes back to the wizard's shoulder. A speak-with-animals spell turns it into an unreliable witness that overheard the count's steward. It can carry a key in its teeth, drop a torch into oil, or chew through a rope at the right moment. None of this is on the stat block, but none of it is overruling the stat block either, because the stat block is essentially empty. The rat is a prop with eyes.

In combat, the rat hides. The wizard's player tucks it into a sleeve before initiative and pulls it back out at the end of the fight. If a hostile spell catches the wizard, the rat takes the splash damage and almost certainly dies, so let that happen with weight. A pet that lives through three sessions has earned a name, a tiny collar, and a moment in session four where it dies saving the bard. Players will remember the rat longer than they remember the encounter that killed it.

Don't roll for the rat in combat. Narrate it. The mechanics are too thin to sustain initiative, and the role is closer to a familiar than to a combatant.

A giant rat is the cellar encounter, the sewer wading scene, the first real fight a level 1 party ever has. CR 1/8, 7 HP, AC 13, no traits and no listed actions in the SRD 5.2 stat block, which means a giant rat is a Bite-and-die mook whose value comes entirely from numbers and terrain. One giant rat is a joke. Six giant rats spilling out of a grain silo with the lights already out is a fight.

Run them as a swarm-by-arithmetic. Spread three to six rats around the party in a tight room so every melee character is engaged and the back rank can't fire arrows without risking the friend in front of them. The 30 ft. walk and 30 ft. climb means rats can flank up walls and across rafters, so attack from the ceiling, drop into the cleric's hood, and force the encounter to fragment. Pack tactics is not on this stat block, so don't paste it on, but standard 5e flanking rules at your table will give Advantage to two rats sandwiching a target if you use that variant.

The damage profile here matters less than the disease implication. Giant rats in folklore carry filth fever, and even though the SRD 5.2 stat block doesn't codify a Diseased rider, you can rule that a PC who finishes the fight Bloodied makes a DC 10 Constitution save or contracts something that needs Lesser Restoration. Telegraph this with prose: the rats are matted, frothing, and the bites burn. Don't surprise the party with a save they didn't see coming.

These rats break and scatter when half their number is dead. Once three of six are down, the rest squeeze through floor cracks and disappear. Don't track them individually past that point. The party should walk out of the silo feeling like they won something small and disgusting, and then notice the bite on the ranger's calf is already turning red.