Giant Weasel

A giant weasel companion is a ranger's beast, a druid's wild shape, or a witch's familiar in practice. An animal handler in town loaned a trained pair. A hunter lent her best mouser for dungeon work. A goblin shaman gifted one as repayment.

Use the weasel to scout. Walk 40 feet, climb 30 feet, Stealth +5, Acrobatics +5, Perception +3, Darkvision 60 feet. The weasel scouts a corridor in one round and a hostile camp in two. Have the handler read the weasel's body language: tail flat means soldiers, ears back means undead, slow blink means inedible food nearby. This gives the party real intel without Pass Without Trace.

In combat, the weasel is a Help action and a tripwire. 9 HP and AC 13 means one hit kills it. Have the handler use it sparingly: dart in to grant Advantage on a critical strike, withdraw 40 feet next turn. If the weasel takes any damage, call it back to a pocket.

The weasel needs food, warmth, hand-scratches. It refuses to fight other weasels or scents that are kin. Give it a name and a tail-twitch the player can mimic. The party will protect it fiercely.

A giant weasel is a CR 1/8 scout alarm, not a combat threat. It has 9 HP, AC 13, Bite at +5 for 1d4+3 piercing, 40 feet walk, 30 feet climb, Stealth +5, Acrobatics +5, and Passive Perception 13. It alerts whatever lives nearby and flees before the party can engage.

Use the weasel as a scouting creature that knows the party is coming. Lead with positioning. The weasel hunts from walls, beams, tree branches using Darkvision 60 feet. When the party enters its territory, it spots them and screams a warning or bolts for the den, using climb speed to take routes the party can't follow. Break line of sight on round one.

If forced to fight, the weasel grants Advantage to actual threats nearby, dives between PC legs using Acrobatics, and darts away with Disengage. It has 9 HP and AC 13, so one solid hit kills it. The weasel flees the moment it takes any damage. Let it go. It has a den to reach and handlers expecting a warning.

The weasel screams once in a horrible high-pitched chitter before disappearing into a wall vent. That sound is the real encounter. The party now knows reinforcements are coming.