Seahorse

A seahorse is set dressing with a stat block. CR 0, 1 HP, swim 20 ft., Stealth +5, no actions, no language. The reason it has a stat block at all is so a druid can Wild Shape into one or so a familiar can occupy a tide pool without breaking the rules. Use it that way and it earns the page.

The interesting use is reconnaissance. A druid Wild Shaped into a seahorse has Stealth +5 (which is real for a Tiny beast), passive Perception 12, and a swim speed of 20 ft. that lets it cling to kelp and watch what the sahuagin are doing without rolling for combat. Don't ask the player for combat tactics. Ask them what they want to learn and let the seahorse drift there. One contested Stealth check against the room's Perception, and either they get the scene or the seahorse gets eaten by the next thing larger than a seahorse, which is everything.

In actual combat, the seahorse does not survive. 1 HP is one bite from anything in the ocean, and the seahorse has no attacks anyway. If the party is using it as a familiar or scout and combat starts, the right call is to dismiss it or let it flee at swim 20 to the nearest cover. A clever player will use it as an alarm rather than a fighter. The seahorse sees the shark, swims back to the boat, the druid drops Wild Shape, the party knows. That's the entire combat utility.

Out of the water, the seahorse dies in a round. Walking speed 5 is also dying speed. If a player insists on bringing one to the surface in a glass jar as a pet, give them the moment, then the slow reveal that they will have to keep changing the water, and let them care for it as a low-stakes side project.

A hostile seahorse is a punchline encounter, but the framing that works is swarm. One seahorse with 1 HP and no attack does nothing. A swarm of two hundred poisonous seahorses summoned by a sea hag's curse, treated as a homebrew swarm with the seahorse as the visible unit, is a real encounter. If your table doesn't run homebrew swarms, the seahorse cannot meaningfully be an adversary, and the honest call is to say so.

The other angle is the seahorse as a sign rather than a fight. The party finds a clutch of seahorses dead on the beach, all facing inland. Something in the deep is pushing things to the shore. The seahorses are the warning, not the threat. Use the stat block to confirm species (Perception or Nature check against passive 12) and let the players draw the conclusion.

If a player insists on attacking a single live seahorse, the result is one dead seahorse and a moment for the table to remember that some encounters do not need rolls. Skip initiative. Describe the swing and the body and move on. Spending five minutes on a CR 0 fight is the table killing its own pacing.

The seahorse is the SRD's reminder that not everything in the bestiary is a combatant. Treat it as the example, not the exception, and use it to set the tone for the next time the party meets something small and harmless and considers the dagger.