Rat

A rat as ally is almost always a familiar (Find Familiar, Wild Shape variants), a witch's messenger, or bound rats kept by a plague-cult NPC who has thrown in with the party. Treat it as a tool, not a combatant.

The actual utility is reconnaissance and access. Climb 20, Darkvision 30, and Passive Perception 12 mean a rat can scout a chimney, vent, sewer pipe, or the gap under a portcullis the party cannot 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 cannot fight. AC 10 and 1 HP mean any spell, arrow, or boot kills it instantly.

If the rat is a Find Familiar, the wizard sees through its eyes as an action and it returns to a pocket dimension when destroyed. If it is a campaign NPC pet, play the loss when it 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. CR 0, 1 HP, AC 10. Bite at +2 doing 1 piercing damage. Agile means it doesn't provoke opportunity attacks when moving away. 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 that a corpse nearby has been there a while.

If you do need it in initiative, the numbers are merciful. Treat any unarmed swat or kick as a successful kill on a hit. Walk 20, climb 20, Darkvision 30, and Passive Perception 12 mean it's good at getting away. 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. A swarm is a different stat block. If the party stomps loudly through a plague district with food in their packs, the decision is whether the noise summons one rat (atmosphere) or twelve (initiative). Telegraph the difference. A dozen rats in the rafters watching silently is a horror beat. A dozen rats screaming down a stairwell is a combat encounter.

Have one rat sit on the corpse of an NPC the party knew, eating something they don't want to look at too closely.