Giant Seahorse

A giant seahorse is an underwater mount and transport. It has CR 1/2, 16 HP, AC 14, a Ram attack at +4 for 2d6+2 bludgeoning (or 2d8+2 if charging), and a Bubble Dash bonus action that moves up to 20 feet without provoking opportunity attacks. The key is 40 feet of swim speed, which moves a mounted rider three times faster than swimming unassisted.

Use the seahorse for travel scenes. The party rides a herd across a sunken city, between reef islands, or down to a shipwreck. The scene is the journey, not combat. A single seahorse carries one Medium rider with a free hand for weapons or spellcasting.

Out of water, the seahorse is fragile. Walk speed 5 feet, no attack, Intelligence 2. On land, it's a rescue mission waiting to happen. In water, it holds position while the rider fights, can flee at 40 feet of swim speed, and tolerates a rider's repeated commands like a horse tolerates its mount. Beasts at Intelligence 2 don't understand tactics, so treat the seahorse as patient and obedient but also skittish.

If combat happens in deep water, keep the seahorse away. 16 HP means a single hostile hit kills it. Have the seahorse Disengage and swim toward kelp or the surface. A dead mount in deep water creates a new problem for the party that isn't fun. Give the seahorse a name before the session. The party bonds with the one that nips at the bard within ten minutes.

A territorial giant seahorse guards a reef, a kelp grove, or nesting grounds. The party swims into the wrong patch and the seahorse charges them as a warning. It's not malicious; it's protective. Sell it as an underwater moose defending its space.

The seahorse has Ram at +4 for 2d6+2 bludgeoning, or 2d8+2 if it charged at least 20 feet before hitting. When it hits, it pushes the target 5 feet. In deep water, being shoved breaks bearing and forces the party to chase falling gear. That's the real threat, not damage. Three rounds of the herd ramming into the party, dropping weapons and spellbooks, and disorienting everyone works as a scene.

Once the seahorses realize the party isn't hunting the nest, they retreat into kelp. Killing one is easy at AC 14 and 16 HP. But any druid at the table will object, and ecology doesn't forget. Reward the party that figures out to leave without being told.

Name the lead seahorse. A territorial animal named Bristle is more memorable than a stat block.