Artist’s description from DA:

Whoo, it’s been a looong while since my last headcanon, hasn’t it? I missed doing stuff like this <3 Anyway, I was in a water mood and decided to draw some sea creatures which of course ended up being pony hybrids… So, in my universe there are three different species of equines that generally live underwater:

Merponies:

Merponies are aquatic mammals characterized by having a pony head and both flippers and flukes. They are not covered in fur, feathers nor scales, but in plain skin, which colors remind those of dolphins, killer whales, yellow tangs, lion fish, clownfish, angelfish and so on. The variety of patterns is so wide that most merponies show absolutely unique colors and designs.

Merponies communicate through a large range of sounds, from whinnies and snorts when they’re on the ground, to acoustically different sounds such as ultrasounds, whistles and cries. They are a quite isolated society which usually prefers not to attack others, but the single individuals are very united and can be dangerously revengeful against those who have betrayed them. They live in shallow waters more or less everywhere, not only in sandy backdropped seas or oceans but also in calm and wide rivers or lakes. Their dwellings are built underwater but at very low deeps. Some even crawl on the ground and hollow some kind of “pools” connected to the sea (or anything else) so that they can stay wet even though they’re not dived. However, since they feel safer, they tend to sleep, eat and generally live as much as they can underwater. They can stay underwater without breathing for a couple of hours or a bit less. This span of time, though, is not enough to sleep, since they need at least 7 or 8 hours of complete rest every day, so they behave like dolphins: half of their brain sleeps while the other half stays active and viceversa. This allows them to be never completely helpless in case of danger or threaten. Their feeding is based off fish and shellfish; they usually hunt in herds so that they can improve their chances of success. Foals start eating fish when they’re 2 years old (quite late compared to their earth siblings), but they learn to get their own food only when they’re 10 or older. 

Merponies can actually stay and move (event though very slowly) on the ground, but for short periods: they need to keep their skin evermore damp, so it’s more likely to see them out of water when it’s raining or during a humid, hot day.

The whole body of a Merpony is conceived to be hydrodynamic, from their head to their pectoral and caudal fins. Some types of Merponies show a crest which goes from their skull to their back (the vividly colored azure one on the left is an example), others have a dorsal fin like most aquatic mammals. A few Merponies are provided with a set of two extra fins between their flukes and their flippers – these are probably the equivalent wings of Pegasi, and they give the Merpony more balance especially in sharp turns or when swimming through strong currents.

The Merponies’ mating rite is something ponies have abandoned (even though hints of it are noticeable in Pegasi’s “love courtship” before a stallion and a mare actually get together). Males of this species draw their female’s attention by twisting, twirling and doing other tricks around the possible mate, until they start to imitate their suitor. Since Merponies are a quite aggressive species, a few of them show battle scars and courtships can be scarily violent. Pregnancy is not painful for mares of this species (unlike for Pegasi and Unicorns), but they usually don’t have more than two foals because it may be difficult to sustain a large family even for a pair of good hunters.

Merponies can mate with ponies, as well. It’s very rare because of the various drawbacks a couple ought to deal with, and because of the general hostility of Merponies towards other species. However, not everything is possible: mares (either Pegasus, Unicorn or Earth Pony) can’t get pregnant from a Merpony stallion. For some mysterious reason that scientists are still studying, they are somehow sterile to the species’ semen, whereas Merpony mares can get pregnant from a pony stallion. The hybrid is usually born as a two-legged Merpony who can live either on the ground or underwater. There are no recorded cases of such hybrids living longer than a few years (the oldest reaching 8 years): every calf developed a mysterious and terminal disease linked to their half “land” blood, but there haven’t been enough cases for researchers to investigate adequately.

Hippocampi:

An Hippocampus is an older subspecies of ponies which, unlike Merponies, has gills and can breathe underwater. Hippocampi use their nostrils to smell underwater, and their ears (which aren’t always external) are among the most sensible of the whole animal kingdom. They’re featured by various fins on their body, not only on their back and tail but also on their legs and sometimes neck and ribs. Their caudal fin is vertical and their internal skeleton is made of cartilage, not bones, so they’re quite light compared to Merponies. Not all of them have four legs like a pony – most only have two and only a few have flippers like Merponies.

They’re much rarer than Merponies and are now almost impossible to find, having suffered the hunting of fisherponies for centuries. They’re still victims of hunts, now illegal, and the few remaining colonies live hidden in caves several feet underwater. They lay eggs and their mating rituals are much calmer than those of Merponies. At the same time, family ties are much less tight, too, and younglings generally become independent quite earlier. Hippocampi feed off fish and seaweed, so they’re not strictly carnivorous, but they can live without food for several days in a row, the strongest males even for a week. 

Kelpies:

Integral part of Equestria’s mythology, these creepy creatures are real existing monsters infesting the shallow waters of isolated swamps and small lakes. They’re only male and, even though they can live up to a couple of centuries, only very few are still alive. Their bodies, sustained both by bones and cartilage, are made of a thick substance vaguely resembling a scaly skin. The crest running down their spine and tail ends with photic parts (like those of fireflies) which attract their prey and act as sign of aggressivity and threat when facing an enemy. Their legs end with webbed calwed paws – the back ones are provided with a rear talon which can secrete venom when stuck in the victim. Kelpies have sharp fangs and a pointy, crooked horn which can actually channel no magic and is maily used as a weapon, similarly to almost all the other body parts. Though this species has gills, he can resist for very long periods of time on the ground as long as it keeps its body wet enough.

Following the terrifying stories surrounding these living legends, Kelpies guarantee the existance of their species by luring mares (Merponies as well as Earth Ponies, Pegasi or Unicorns) and impregnating them, keeping them in their territory until they lay the Kelpie’s eggs. Kelpies can also shapeshift, similarly to Changelings, and can assume the looks of a handsome young stallion to attract the mare they’ve aimed at. If the prey doesn’t fall into the trap, she’s captured and harassed as soon as the Kelpie finds her alone in his territory. It’s hard to understand whether such legends are true or not, but these bloodcurdling tales are enough for most ponies to keep away from undiscovered, wild marshes and bogs.