Rat

A rat as ally is almost always a familiar (a wizard's Find Familiar conjures one, and the party druid can briefly become one via Wild Shape variants), a witch's messenger, or a colony of bound rats kept by a plague-cult NPC who has thrown in with the party for their own reasons. Treat it as a tool, not a combatant.

The actual utility is reconnaissance and access. Climb 20 plus Darkvision 30 plus Passive Perception 12 means a rat can scout a chimney, a vent, a sewer pipe, or the gap under a portcullis the party can't fit through. It can carry a folded message between rooms, drop a small key into a lock from above, or chew through a rope at the GM's discretion (the rat has no listed bite damage in its action list, so adjudicate the chew as a slow downtime action rather than an attack roll). The rat cannot fight. AC 10 and 1 HP mean any spell, arrow, or boot kills it instantly, so do not send it past a creature that has any reason to swat at moving things.

Familiar rules vary by edition and table. If the rat is a wizard's familiar via Find Familiar, the wizard sees through its eyes as an action and the rat returns to a pocket dimension when destroyed. If the rat is a campaign NPC pet, play the loss when it inevitably dies. The cleric will hold a tiny funeral.

Give the rat a name and one specific behavior. It always sits on the wizard's left shoulder. It always squeaks twice when food is poisoned. The quirks are the whole character.

A single rat is not a fight. It is a prop, a sound effect, a way to make a basement feel inhabited. Use it as ambient texture in a sewer scene, as a familiar's distant cousin scuttling across a kitchen floor, or as the first warning sign that a corpse around the next corner has been there a while. The stat block exists so you can resolve the rare moment a player insists on rolling against one.

If you do need it in initiative, the numbers are merciful. AC 10, 1 HP, no listed actions. Treat any unarmed swat or kick as a successful kill on hit; the rat dies to a dropped book. Walk speed 20, climb speed 20, Darkvision 30, and Passive Perception 12 (Perception +2) mean it's good at getting away. If a wizard's Mage Hand harasses it or a rogue tries to grab it for an ingredient, give it Dex save +0 and let the dice decide. The interesting friction is whether the party catches one alive, not whether they win the fight.

Where rats become a real threat is in numbers, and that's a swarm of rats, a different stat block. If the party stomps loudly through a plague district with food in their packs, the GM-side decision is whether the noise summons one rat (atmosphere) or twelve (initiative roll for a swarm). Telegraph the difference. A dozen rats in the rafters watching silently is a horror beat. A dozen rats screaming and pouring down a stairwell is a combat encounter, and you should swap the stat block accordingly.

Have one rat sit on the corpse of an NPC the party knew, eating something they don't want to look at too closely. The image does more work than any roll.