Weasel

A weasel is a familiar, a thief's pocket companion, a druid's smallest wild shape, a kid's pet that moved in for an arc. The stat block has no actions because a weasel cannot meaningfully damage anything. AC 13, 1 HP, 30 ft. walk and 30 ft. climb, Stealth +5, Acrobatics +5, Perception +3 with passive 13, darkvision 60 ft. Run it as a sensor and a cute escalation device, never a combatant.

In play, lean on the size and the stealth. A weasel slips through gaps the rogue can't fit through, climbs a chimney to confirm whether the room above is occupied, sits in the rafters during a tense negotiation and reports back via telepathy if it's a familiar. With darkvision 60 ft. it scouts dark rooms the party would otherwise need a torch for, which is the difference between a clean infiltration and a botched one. Acrobatics +5 means it sticks landings the party fighter cannot.

The weasel does not fight. Even a stiff breeze rolls one die against its 1 HP. If a player insists on combat, point at the stat block, then describe the weasel hissing dramatically and fleeing into the nearest pocket. As a wizard's familiar it should be dismissed back to its pocket dimension the second initiative rolls; the spell exists for a reason. As a non-magical pet it goes in a sleeve, a satchel, a hat. Anywhere but the floor of an active fight.

Give the weasel a name and one specific bad habit nobody at the table predicts. Steals coins, bites the bard's earlobe, refuses to drink anything but goat's milk. The weasel is comic relief that earns its keep in a single tense scouting run per session.

A hostile weasel alone is not an encounter, so don't run one. Run a hag's familiar, a witch-king's spy, a dire weasel's lesser kin scouting for the pack, or a swarm-of-weasels narrative beat where the threat is the diversion they create rather than the bites they land. One weasel has 1 HP and zero attacks; whatever the weasel is doing matters more than what the weasel is.

Use it as surveillance. The party notices the same brown weasel watching from the inn's roofbeam, then again at the duke's stable, then in the chapel where they hid the relic. With Stealth +5, passive 13, and 30 ft. climb the weasel stays just out of casual reach, and any attempt to swat it ends with the weasel two beams over. Killing it accomplishes nothing in the moment and a great deal afterward, because whoever was scrying through it now knows exactly what the party looks like and that they are willing to crush small mammals for asking questions.

If the players catch the weasel, decide what it actually is before they do. A familiar's controller takes psychic damage when it dies, which the party's wizard will weaponize the moment they realize. A polymorphed scout reverts on death, leaving a tiny human corpse and a much larger problem. A regular weasel that was just a weasel means the party has been paranoid for two sessions, which is its own kind of payoff.

Don't roll initiative against a weasel. Resolve any chase as one Dex (Acrobatics) check against the weasel's passive 13 plus terrain, and let the result steer the next scene.