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).

Use the size and strength. Strength 24 means the mammoth pulls weight no horse team can match: a siege ram, a stuck wagon, a fallen tree across the road, a portcullis that needs lifting. In overland travel it carries Large gear and Medium passengers easily. The 50 ft. walk speed 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 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. Use two Gore attacks on targets the handler designates, and Trample the first creature knocked Prone.

The mammoth is bad at delicate scenes. It cannot enter most buildings and the smell alone spooks every horse and stray dog for a quarter mile. A party that brings a mammoth into a town has chosen to be visible. 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 attack. Multiattack is two Gore attacks at +10 doing 2d10+7 piercing each. If the mammoth moved 20 feet straight toward a target before the hit, Gore also knocks the target Prone. On a bonus action it can Trample a Prone creature within 5 feet, a Dexterity save DC 18 taking 4d10+7 bludgeoning on failure or half on success. The combo is Gore into Prone, then Trample next action for 29 average damage on the failed save.

The party's options are kill it fast (126 HP and AC 13 is one good round of focused damage at level 5 or higher) or get out of the lane. 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. Retreat happens fast. A bloodied mammoth that is no longer near whatever angered it turns around and walks away. Beasts protect a thing, and once the thing is safe or dead, they stop.

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