Underwater Equines
Another headcanon drawing, one I wanted to do for some time. Originally I wanted to add all seacreatues, but in the end I decided to focus only on the "sea equines".

Despite the title, a lot of these creatures are not related to ponies or any equine creatures of the land.

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Siren

Biggest of these creatures. They use their serenade to mesmerize their targets and steal their magic. The gem on their chest is either the source of their powers or just amplifies it. There are theories that sirens were mermares in the past, but got corrupted. They are able to breathe both in water and on land.

Seahorse

Next to sirens, seahorses are the only "sea equines" who can use magic. When they do, the spot on their cheek lights up (with a different color for each seahorse). Their appearance can look vastly different, depending on how deep they live in the oceans. Similar to the seahorses of Earth, in a relationship the male seahorse carries out the offspring. They are only able to breathe underwater.

Seapony

Today known seaponies are not actual real ones. They are hippogriffs who are able to take on a seapony form thanks to a magical pearl that may have been connected to the actual seaponies. It's unknown if seaponies are still around or are long gone. Or if they even looked like today's 'seaponies'. As hippogriffs they are distant relatives of ponies. They don't have magic on their own, they can only transform if they have a piece of the magic pearl.

Because they can take on their original form they can easily live on ground, however if they don't want to, they can still breath on land, but their bodies need to be in water, because their skin dries up fast. Underwater they can breathe because of the magic of the pearl, unlike the other "sea equines", who most of the time have gills.

Mermare

Second biggest among these creatures (their female members are bigger than male seaponies), they are also the most colorful. They have manes, however their tails are fin, but mimic their mane. Because of their similarities, like their size and how they like to sing, many times being mistaken for sirens. Because of that, there were many cases where they were attacked or chased away. They can breathe both on land and in water.

Orcapony/Sharkpony

Maybe the only ones closely related to ponies (and the closest to earth ponies). They may have been earth ponies that turned to the sea for one reason or another. Both creatures are not as big as their name counterparts. The symbols on their back are unique and gain it over their life growing up. Despite their name, orcaponies not all look like orcas, some mimic other whale/dolphin species (names come from the fact the orca look is the most common one). So are sharkponies have designs mimicking different sharks. Only orcaponies are able to breathe on ground, but not for a long time underwater, while sharkponies only in water.

???

There are things living in deep parts of the ocean. Maybe they should stay there.