Mammoth

A mammoth on the party's side is a mount, a barbarian tribe's gift, a frontier lord's pulling beast, or a druid's summoned guardian. The mammoth has Int 3 and no languages, so the ally framing always rides on a handler. Pick whether the mammoth is bonded to a single PC (mount) or to an NPC who is loaning it (gift, hire). The two play very differently.

Use the size and the strength. Strength 24 means the mammoth pulls weight that no horse team can match: a siege ram, a stuck wagon, a fallen tree across the road, a portcullis that the party needs lifted. In overland travel it carries Large gear and Medium passengers easily. The 50 ft. walk speed is not fast for a mounted scene, but it is steady, and the mammoth does not tire on the schedule a horse does.

In combat the mammoth is a wall the party stands behind. Huge size blocks line of sight, AC 13 with 126 HP soaks several rounds of attention from low-CR enemies, and the trample threat keeps anything Medium-sized respectful of its space. Position it across a doorway or in a treeline and let it absorb the opening volley. It will not coordinate. It hits whatever the handler points at, and it keeps hitting until the target drops or the handler signals otherwise.

The mammoth is bad at delicate scenes. It cannot enter most buildings, and the smell alone spooks every horse and stray dog within a quarter mile. A party that brings a mammoth into a town has chosen to be visible. Treat that as a feature. The villain knows the party has arrived because someone sent word that a mammoth is tied up outside the inn.

A mammoth is a Huge angry hill that moves 50 feet a turn. The encounter design question is what the mammoth is doing here at all, because mammoths do not stalk parties. They are charged by, stumbled into, or angered by something the players did three rounds ago. Sell the cause: a calf the party walked too close to, a bull in rut, a herd disturbed by hunters who have already fled. The mammoth is not the villain. It is the consequence.

In motion the mammoth is a charging trample. 50 ft. walk plus Huge size means it threatens a 15-foot front and covers most of the battlefield in two turns. Strength 24 with the +10 save means the standard knock-prone or shove answers do nothing. The party's plan needs to be either kill it fast (126 HP and AC 13 is one good round of focused damage at level 5) or get out of the lane. There is no third option for a melee-heavy party that wants to "stand and tank."

Use the Huge size as terrain. The mammoth blocks line of sight for Medium creatures behind it. A wizard can hide behind the mammoth from an archer on the far side, or a rogue can dart between its legs to disengage. The party should figure this out in round two. Reward whoever calls it.

Retreat happens fast. A bloodied mammoth (63 HP) that is no longer near whatever angered it just turns around and walks away. Beasts don't avenge. They protect a thing, and once the thing is safe or dead, they stop. If the party stops chasing, the mammoth stops fighting. The encounter ending should feel like both sides agreeing that the disagreement was a misunderstanding, written in 800 pounds of broken ribs.

Have the ground shake before anyone sees the mammoth. Three thumps, a pause, then the trees come apart.