Polar Bear

A polar bear ally is a druid's wild shape, the bond-beast of an arctic ranger, or a cub the party rescued and that has grown up alongside them. The framing matters: a wild polar bear does not befriend humanoids, but a hand-raised one will defend its pack to the death and its pack is now the party.

In combat, run the bear as the front line. Improvise a Multiattack of Bite at +7 for 1d8+5 and Claws at +7 for 2d6+5 (the SRD does not list these explicitly, so use these numbers and tell the players that is what the bear does). 42 HP and AC 12 means the bear can hold a chokepoint for two or three rounds against tier-1 enemies. Use it to plug doorways the rogue is unlocking behind, or to wade into the river and pin the goblin canoe so the ranger can shoot.

Out of combat, the polar bear is a heavy that lives in the cold. Walk 40, swim 40, and Cold resistance lets it cross winter terrain the party would otherwise need rope and pitons for. Use it as a search-and-rescue beast: Perception +5 and the ability to break thin ice with its bulk lets it find a PC who fell through and dredge them out. Players will treat the bear as a member of the party once it has saved one of them.

The bear eats a lot and does not enter buildings smaller than a temple nave. Plan for the scenes where it has to wait outside, and have it greet the party at the gate when they come back out, having killed and eaten something the gatekeeper now wants compensation for.

A polar bear is a CR 2 ambush in the snow, and the ambush is the encounter. Stealth +4, Perception +5, white fur on white tundra: the party walks past a snowbank and the snowbank stands up. With 42 HP, AC 12, and Strength 20, this is a real fight for a tier-1 party, especially when it is hunting and they are between the bear and a meal. The stat block does not list actions, so improvise a Multiattack of one Bite at +7 for 1d8+5 piercing and one Claws at +7 for 2d6+5 slashing, in line with what brown bears do in 5e. Do this before the session, not at the table.

The bear opens with a charge. Walk 40, swim 40, and Cold resistance means it is faster than the party in any winter terrain that matters. Round one is a Bite on the closest PC, and on a hit the bear has them in its arms. Round two is Claws on the same target while it stands over them. The bear does not switch targets if its current meal is still moving. This is what makes a polar bear scary: it commits.

Use terrain. A fight on a frozen lake is a different fight than a fight in a forest. Run thin ice (DC 12 Strength save when the bear charges across it, fall through on a fail, then a turn climbing back up while the bear circles in the water). The party that is not prepared for cold-weather adventuring learns the difference between adventuring gear and survival.

A polar bear retreats once Bloodied and the prey is fighting back too hard. It is a predator, not a soldier. Have it back off, climb a snowdrift, and watch from cover. The party should know it is still up there. The next watch should hear it breathing.