Seahorse Steed

Sometimes seahorses are portrayed as the underwater equivalent of horses, and are can be ridden by characters. This is particularly entertaining, since Real Life seahorses are very, very small, the largest not exceeding eight inches in length, and as slow as a fish can get without being totally immobile.

Examples of Seahorse Steed include:

Classical Mythology

Older Than Feudalism: Poseidon was often shown with his chariot pulled by horses or seahorses (as King Triton does in The Little Mermaid).

Comic Books

Aquaman sometimes rides a giant seahorse, named Storm (in the Silver Age, anyway).
A Disney Comics story where Donald Duck, Scrooge, and Gyro discover an underwater kingdom near one of Scrooge's drilling sites. The seapeople had seahorse racing.
- Similarly, an episode of DuckTales (1987) where Scrooge, Gyro, and Doofus visit Atlantis had all of the Atlanteans use seahorses as mounts.

Film - Animation

There is a seahorse race in DreamWorks' Shark Tale.
In Pinocchio, Jiminy Cricket rides what appears to be a sea donkey.
My Little Pony: The Movie features SeaPONIES.

Tabletop Games

In Dungeons & Dragons, aquatic elves and locathah sometimes capture and train Giant Sea Horses and use them as steeds.
The same for Slaanesh demonic steeds in Warhammer 40,000

Video Games

In King's Quest II, Graham does this. They even have saddles. Do the mermaids ride them sidesaddle or something?
Naija can do this in Aquaria.
The city-building game Atlantis Tycoon has a building (The "Atlantean Castle") that spawns merman knights that ride seahorses to protect your city.
The Pokémon Horsea, despite being only a little over a foot long, can be taught the move Surf via TM, which lets it ferry it's trainer around. Not quite straight since surf's out of battle effect creates a shark/whale thing for the trainer to ride on, you're not actually one your Horsea itself.
An early quest in Vashj'ir allow the character to obtain such a mount.

Web Comics

Eridan of Homestuck, as seen here.

Web Original

In the credits of Yahtzee Croshaw's Zero Punctuation "review" of Duke Nukem Forever, he's riding a seahorse, (with Jimi Hendrix?).

Western Animation

SpongeBob SquarePants befriends a sea horse he calls Mystery.
In Help! I'm a Fish, Sasha the seahorse helps carry the main characters around. In the end, she is turned into an actual, land-bound pony.
Naturally, The Little Mermaid: The Animated Series had Ariel tame an uncontrollable purple seahorse named Stormy. Other episodes had merpeople riding seahorses as well.
- Averted with Herald the seahorse, however.