Baboon

A baboon ally is a stretch but workable: a druid's Find Familiar variant in a homebrew setting, a circus animal the party rescued, or a troop the party fed during a famine that now follows them through one specific jungle. The framing is that the baboons aren't pets and aren't trained. They tag along because the food is good and the humans are amusing, and they leave the moment something better comes up.

Mechanically there is almost nothing here. CR 0, no actions, 3 HP. Don't roll initiative for them. Use the troop as a scouting and distraction asset out of combat: a baboon can climb to the top of a tree and screech if it sees armed men on the road, which is functionally a free Perception check on the party's behalf. Inside a dungeon, a baboon can squeeze through a gap the dwarf can't and trip a lever or carry a small object across a pit.

In combat, have the baboons run interference rather than attack. Describe one of them landing on a bandit's head and clawing at his eyes, granting the rogue advantage on the next attack via the Help action. If the bandit hits back, the baboon dies in one shot, and the party should feel that. The death of a friendly baboon is a small grief that costs you nothing and earns you table buy-in for the next animal companion scene.

Let one specific baboon have a name and a habit. The one that always steals the wizard's hat. When that baboon dies in a goblin ambush three sessions later, the wizard will mourn it more than they mourned the last NPC you wrote two pages of backstory for.

A single baboon is texture, not a threat. With 3 HP, AC 12, and no listed actions, one baboon dies to any attack roll that connects. Use them in groups of six to ten as ambient hazard or comic obstacle, not as a fight the party is supposed to take seriously. Their job is to steal the rogue's hat, scatter the camp, and let the players feel like the wilderness has its own opinions.

Run them in a troop. Climb 30 ft. plus walk 30 ft. means baboons drop from canopy onto bedrolls, snatch a torch off the wall, and skitter back up out of melee reach before the fighter's turn comes around. Treat improvised attacks as 1d4 bludgeoning at +1, but more often use the Help action and Shove rather than damage. A baboon clinging to the bard's face is a problem the bard has to solve, not a roll the GM resolves. Passive Perception 11 is just enough to catch a sneaking party member if they roll badly.

Baboons rout the moment two of them go down. Wisdom 12 with no Frightened immunity means a Loud Noise, a Thaumaturgy, or a single Burning Hands sends the troop screaming back into the canopy. Don't make the players grind through ten 3-HP statblocks. The combat ends when the baboons decide the cost is too high, which is fast.

Have one baboon make off with something the party actually needs. Not the longsword, the spell component pouch. The chase scene through the trees is the encounter. The baboon doesn't even fight back, it just runs, and the party has to figure out how to bring it down without killing it (or accept losing the components for a week).