Scorpion

A scorpion as ally is a stretch, and the workable frame is the desert druid's familiar. A druid or warlock with Find Familiar can take a scorpion form, which gives a Tiny scout with Blindsight 10 ft., 10 ft. walk speed, and a body that does not look like a familiar to enemies who have never seen the spell. In a setting where every wizard's owl gets shot down, a scorpion familiar is invisible by default.

The play is reconnaissance and ambush. The scorpion familiar tucks into a saddlebag, into a guard's boot, into the cracks of a wall. It uses the Help action against creatures within 5 feet, so the rogue with Sneak Attack stationed near the same target gets Advantage from a familiar nobody noticed was there. If the wizard delivers a touch spell through the scorpion, that touch comes from inside the target's clothes, which is a rules-correct horror story.

Outside the familiar frame, a scorpion ally is a curio. The desert nomad keeps a tame scorpion in a small clay jar as a household ward, the alchemist milks one for the venom that goes into anti-venom, the spy carries one as a deniable assassination tool. None of this is on the stat block, but the stat block is essentially blank, so the GM has full license to give the scorpion a single specific narrative purpose and let it be that.

Don't roll for the scorpion in combat. Narrate it, the same as any prop. Its 1 HP means anything within 5 feet kills it on contact, and that loss should land as a small grief, not a missed turn.

A scorpion is a CR 0 prop that exists to tell the party they are in the desert. Tiny size, 1 HP, AC 11, 10 ft. walk speed, no traits and no listed actions. There is no fight here. The scorpion is environmental texture, the rasp under the bedroll, the thing the rogue almost stepped on while picking the lock on the tomb door.

Use scorpions as low-stakes hazards in box text. The party makes camp in the dunes and a scorpion is in the boot the wizard left out. The thief reaches into a wall niche for the gem and a scorpion clatters out. None of this requires initiative. Call for a Dexterity save against an arbitrary low DC (10 or 12), and on a failure the PC takes 1 piercing and a small dose of poison that costs them an hour of itchy discomfort. The scorpion is the punctuation mark, not the sentence.

If you want a scorpion to be threatening, use it in numbers, in a confined space, with the lights out. Twenty scorpions pouring out of a sand-buried urn at midnight is a Dexterity saving throw to dance back, an Athletics check to clear the boot, and three rounds of mild paranoia. Resolve the whole thing as a hazard rather than as combat. The scorpion stat block is too thin to sustain initiative against a party of any level.

Larger predators eat scorpions, so if a giant scorpion or a sand worm is in the encounter, sprinkle the small ones around as set dressing. The party will draw the connection without you saying it: this is a place where things hunt other things, and we are smaller than both.

Have a scorpion crawl across the open spellbook the wizard left on the breakfast rock. The player will pick it up before reading the spell. That delay is the entire scene.