Brown Bear

A brown bear on the party's side is the druid's bread and butter: animal companion, summoned beast, conjure animals output, or wild shape. Less commonly it is a beast tamed by a frontier hunter or a circus, or in stranger games a bonded familiar that grew up. Pick the framing and commit, because the bear has Int 2 and no language. The "ally" is really whoever stands next to it.

The bear's job in a fight is grappling, blocking, and absorbing one bad hit so a player doesn't. With AC 11 and 22 HP it folds against anything CR 2 or up, so don't put it in front of a real threat. Use it on goblins, wolves, brigands, the kind of mooks that can't burst it down in one round. The 30 ft. climb is the underrated tool. Send the bear up after the kobold archer on the rooftop. Send it up the cliff after the fleeing villain.

Out of combat the bear is a tracking machine and a deterrent. Passive Perception 13 plus Darkvision 60 ft. means it notices things the party misses on watch, and a bear at the campfire keeps wolves and bandits at the edge of firelight rather than in it. Reward the party with quiet nights when the bear is with them. Make the nights it isn't there feel different.

When the bear takes its first serious wound, have the bonded character feel it. The link between druid and beast is part of the relationship, and one wince at the wrong moment sells the whole thing.

A brown bear is the wilderness encounter that keeps a low-level party honest. The SRD 5.2 stat block has no listed traits or actions, so you are running it as a Large beast with the unarmed strike rules: a Strength-based attack at +5 to hit for roughly 1d8+3 bludgeoning, or borrow the older edition's claw and bite if your group prefers two attacks. Pick a version before initiative and stay with it. With 22 HP and AC 11 the bear folds quickly to focused fire, so the drama is in the first two rounds.

Open with the bear already on top of the party. Brown bears do not stalk like cats. They blunder out of brush at 40 ft. of walk speed, and Passive Perception 13 means it almost certainly noticed the party's campfire well before initiative. The first round should put the bear in melee with whichever PC was on watch, which is usually the one with the worst Constitution save. The 30 ft. climb speed is the surprise. A treed party member is not safe. Have it scramble up after them on round two.

The bear is unaligned with Int 2. It is not malicious. It is hungry, or defending cubs, or surprised. If the party drops bait (rations, a downed pack horse, a screaming halfling someone has shoved away from the fire) the bear takes the bait over the fight. Reward parties that think laterally. Once it drops below half HP and there is meat available that isn't fighting back, the bear disengages and goes for the easy meal.

Have the bear bite first and roar second. The roar should land a beat after the first hit, when the party is already reacting. A bear that announces itself wants to be respected. A bear that just attacks wants to eat.