Crab

A crab "ally" is almost always a familiar or a beast bond. A wizard with the Find Familiar spell can pick the crab form, and a coastal druid or ranger may have one as a companion or a friendly visitor at camp. Treat it as a prop with personality. The crab cannot fight for the party in any meaningful way (3 HP, no listed attack, +0 to-hit if you improvise one), but it can scout, fetch, and provide flavor.

The useful trait is Blindsight 30 in murk where the party cannot see. A familiar crab in a flooded dungeon swims ahead at 20 feet, returns, and tells its master what it sensed. A druid's crab buddy notices the corpse on the seabed before the cleric's diving bell does. Use the crab's senses to deliver one piece of information per scene, and let the player feel clever for sending it.

Out of combat, the crab is comic relief and a touchstone for the character who owns it. Name it. Give it a habit (always finds the shiniest pebble in any room, hates the bard, sleeps in the wizard's hood). Players will remember the crab longer than they remember the warlord they killed last week.

If a fight breaks out, the crab hides under something. Do not put 3 HP on the front line for the sake of "everyone fights together". The crab survives by being too small to matter, and that is its character.

A crab is a CR 0 set piece, not a fight. With 3 HP, AC 11, no actions in the stat block, and a 20-foot walking and swimming speed, a single crab dies to one cantrip and contributes nothing to a real encounter. Use it for atmosphere: tide pools that scuttle when a torch passes, a beach the party crosses at low tide, the kitchen of a coastal inn. Blindsight 30 means the crab notices the party before they notice it, which is the whole drama.

If you want crabs to matter mechanically, use a swarm. Twenty crabs around the party's ankles is difficult terrain on a beach and a slow morale breaker for a player who hates pinching. Run them as a hazard, not as initiative entries: a Dexterity save (DC 10) when the party tries to sprint, with failure costing 5 feet of movement and a single point of piercing damage. That feels like crabs without making the table sit through twenty +0 attacks that miss everything.

Where crabs work as a real encounter is dressing for something else. A drowned sailor's body on the rocks, alive with crabs that scatter when the rogue rolls it over to find the locket. A trail of pinching tracks that leads the party to the cave they actually need to enter. The crab is the breadcrumb, not the meal.

If a player insists on rolling initiative against a single crab, let them. The crab makes one unarmed-style swipe at +0 (call it 1 piercing), misses on anything but a 20, and dies the next round. Spend thirty seconds, move on. Do not make this a fight.