Giant Weasel

A giant weasel as a companion is a ranger's beast, a druid's wild shape, or a witch's familiar in everything but type. The framings that hold up: an animal handler in town has a trained pair, a hunter has lent the party her best mouser for the dungeon crawl, or a goblin shaman the party rescued has gifted them his old hunting weasel as repayment.

Use the weasel for what it is good at, which is moving quietly through tight spaces and reporting back. Climb 30 ft., walk 40 ft., +5 Stealth, +5 Acrobatics, +3 Perception, and Darkvision 60 ft. mean the weasel can scout a corridor in a round and a hostile camp in two. Have the handler describe what the weasel reports in body language: tail flat means soldiers, ears back means undead, and a slow blink means it found food the party probably should not eat. This kind of scene gives the party real intel without burning a Pass Without Trace.

In combat, the weasel is a small Help action and a tripwire. It is 9 HP and AC 13, so it dies to one bad round. Have the handler use it sparingly: dart in to grant Advantage on a single critical attack, then withdraw 40 ft. on the next turn. If the weasel takes any damage, the handler should call it back to a pocket or pack immediately.

The weasel asks for nothing except food, warmth, and the occasional hand-scratch under the chin. It refuses to fight other weasels, including dire weasels and shapechangers it can smell are kin.

Give the weasel a name and a tail-twitch the player can mimic at the table. The party will protect that weasel with their lives.

A giant weasel is a scout problem, not a combat problem. With 9 HP, no actions in the stat block, and a 5-foot mouth that does not even have a listed bite in the SRD entry, this thing is not going to kill the party in a stand-up fight. What it does is ruin the party's ambush, alert the rest of the dungeon, and then vanish before initiative ends. Run it as the alarm bell of whatever lives nearby.

Lead with positioning. The weasel has 40 ft. walk, 30 ft. climb, +5 Stealth, +5 Acrobatics, Darkvision 60 ft., and a passive Perception of 13. It hunts from a wall, a beam, or a tree branch. When the party enters its territory, it spots them on the GM's free Wisdom (Perception) and immediately moves to either screech a warning to its handlers or sprint back to the den. Have it use the climb speed to take a route the party cannot easily follow, then break line of sight on round one.

In the rare case the weasel has to fight, treat it as a Help action machine for the actual threats nearby. With a +3 Dex and +5 Acrobatics, it dives between PC legs to grant Advantage to the goblin or wolf it works with, then darts back out using the Disengage action. Without a printed bite or claw, you are within rules-as-written to give it an improvised d4 attack at +2 if you need it to close out a kill, but the better play is to keep it scouting, biting once for narrative flavor on a downed enemy, and otherwise refusing to die in melee.

The weasel flees the moment it takes any damage. It is a 9-HP beast with a survival instinct and a den to run home to. Let it.

Have the weasel scream once, in a horrible high-pitched chitter, before it disappears into the wall vent. The party will know reinforcements are coming. That is the encounter.