Giant Seahorse

A giant seahorse is a mount, not a fighter. It has no attacks in its stat block, AC 14, 16 HP, and 40 ft. of swim, which makes it a CR 1/2 underwater horse for a single Medium rider. Run it as transportation and atmosphere. The party rides them across a sunken city, between two reef islands, down to a wreck, and the scene is the journey rather than the combat.

Out of the water, the seahorse is helpless. Its walk speed is 5 ft. and it has no real means to fight on land, so any encounter that drags it onto a beach or a ship's deck is a rescue mission for the party. Inside the water, it's quietly excellent: 40 ft. swim moves the party three times faster than a human PC unaided, the rider can shoot or cast at full effectiveness, and the seahorse holds station while the rider does work. Beasts at Int 2 don't take complex commands, so play it like a horse with the same patience and the same panic responses.

If a fight breaks out, the seahorse's job is to not die. 16 HP and no attack means it's a single hit from a hostile predator. Pull the rider off into combat and let the seahorse hide in kelp, or have the seahorse Disengage and swim straight up to the surface. A dead mount strands the party in deep water, which is its own encounter and not a fun one. The party should spend a moment training the seahorse to come when called, since "the horses are gone" is the underwater version of every western disaster.

Pick names and personalities for the herd before the session. The party will bond with the one that nips at the bard's hair within ten minutes.

A hostile giant seahorse is a stretch, but it works as a territorial defender of a kelp grove or a reef nest, especially during mating or with eggs nearby. The framing isn't malicious, it's protective: the party swims into the wrong patch of seabed and a Large beast charges them with a body-check. Sell it as the underwater equivalent of a moose stomping through a campsite.

Mechanically the seahorse has no listed attack, so improvise from its stats: a STR 15 ram for 1d6+2 bludgeoning is reasonable as a one-off shove, and any hit should push the target 5 ft. with a Strength save against a sloppy DC like 12. The point of the encounter is not damage; it's disorientation. PCs in deep water who get bumped lose their bearings, drop a held weapon, and have to chase a sinking spellbook. That's the scene. Three rounds of the herd ramming into the party and the party deciding whether to fight back or just leave.

Once the herd realizes the party isn't actually after the nest, the seahorses retreat into the kelp and watch from cover. Killing one is easy at AC 14 and 16 HP, but it tilts the local ecology against the party and any druid in the group will have feelings about it. The right answer is to swim away. Reward the party that figures that out without having to be told.

If you do run this, name the lead seahorse. A territorial animal is more memorable as Bristle than as a stat block.