Giant Rat

A giant rat as companion is a wizard's sewage-level familiar that the party refused to abandon. It has 7 HP and a Bite for 1d4+3 piercing, but those numbers don't matter. The rat is a prop that helps with scenes, not a combatant.

Use it for utility and charm. The rat squeezes through a grating, brings back a key or a note, and returns to the wizard's shoulder. It can scout a room by running ahead, deliver a message (via speak-with-animals), and climb into spaces no humanoid can access. In combat, it hides. The wizard tucks it away before the fight and brings it back afterward. If a spell catches the wizard during combat, let the rat take some splash damage. It almost certainly dies, and that death carries weight if the party has warmed to it.

The mechanics are too thin for the rat to roll initiative as a real combatant. Narrate its actions instead. Give it a name, a feeding quirk, a personality. Players bond with named pets more than with numbered creatures. A rat that survives three sessions has earned a moment in session four where its heroism saves someone else, and the table will remember that rat longer than they remember any encounter.

A giant rat is the cellar encounter, the sewer ambush, the first real fight a low-level party faces. It has 7 HP, AC 13, a Bite attack at +5 for 1d4+3 piercing, and Pack Tactics to gain Advantage when an ally is within 5 feet of its target. One giant rat dies in a single round. Six spilling from a silo is a real fight.

Run them by numbers and position. Spread three to six rats around the party so melee characters are engaged and the back rank can't shoot without hitting allies. The 30 feet of walk and climb speed means rats attack from rafters and walls. Have them drop from the ceiling and cling to the walls, forcing the party to fragment their response. Pack Tactics means every rat that works with a friend gets Advantage on its attack. Two rats flanking a fighter work better than four rats attacking separate targets.

The real threat here is not individual damage but coordination and numbers. Each rat dies quickly, but they die in sequence while their packmates keep attacking. Once half the rats are down, the rest scatter and flee. Don't track them past that point. The party wins, but they take damage and see teeth marks everywhere.

Establish the space before combat begins. Is the party in a tight basement where they can't maneuver? A sewer tunnel? An enclosed space makes rats an encounter rather than a nuisance. A wide-open field with the same rats is trivial.