@Background Pony #217B  
Have seen the screencaps of the sirens, as these open their mouths to sing out their attacks, one could see a number of thin and irregular needle-like sharp teeth. Much smaller opening between the teeth, but still suitable for snatching fast-swimming fish into the water. But they seemed to have a pair of incisors at the very front, just like a rabbit. What kind of function can this have? Maybe to bite off large pieces of meat on large animals while the many thin teeth on the sides would make sure to catch fish very quickly. But some of the teeth are reminiscent of those found on the ponies, even though they do not have molars. They have hooves on their arms, which can be used to crush objects or food such as shellfish.
 
This means that the sirens had a variety of food sources at sea as they do not specialize in specific foods, and certainly have very high bite strength. Currently they are more reminiscent of the mosasaurs - some of the most bloodthirsty predators in the oceans in history. I like the design by Tony Fleecs on the sirens which thus become very elegant sea creatures that can swim very fast, and like Sourcherry think their teeth are meant to catch fast fish. Obviously, the sirens were very dangerous predators as with the mosasaurs, which could attack ponies and anything else considered prey.
 
The dazzlings are something special as they had gems united with their bodies and thus can absorb the emotional magic they provoke with their song, so they can fly in the air without the use of wings - these are possibly sirens that had modified themselves rather than a “natural” phenomenon as those who knew the sirens did not know what the dazzlings were capable of.
 
All other pony-based life forms in seawater must be either predators in deep sea or herbivores in shallow waters such as in freshwater, such as the classic seaponnies, the hippogriffs / seaponnies and the kelpies which are probably far closer to the sirens than the seaponnies. The kelpies are a bit whimsical, whether they are sea creatures or water elements.

@Background Pony #217B  
It is just my headcanon. It’s not like NMM had Hanya-mask inspirations either, mostly I just got bored of drawing same fangs all the time.

@FanOfMostEverything  
Although the sirens’ teeth don’t quite look like that in the series, and it’s not like the show was afraid to show sharper fangs.

@SourCherry  
The Storm kind can be difficult, as we have only seen a lone creature with a visible mouth, the Storm king that is larger than the rest of his kind.
 
Today, we have been able to examine the food habits of wildlife and thus can conclude that very few animal species are either true predators or herbivores, as most are opportunistic in terms of food intake. Omnivores to a greater or lesser degree seemed to be the rule among mammals, you can have 90% meat and 10% plant or vice versa as seen with the horses and ponies. Protein is important for energy production, and for the pegasi it is necessary for them to be able to obtain energy-rich foods. They can not grow anything on the clouds and thus have to pick up the food from the ground.
 
After looking a little more at the screencaps, it is clear that the ponies have canines - which are a little further back in the row of teeth. They only appeared as an expression of aggression, when the ponies clenched their teeth and bared them. The stallions, mares and alicorns have these. As they had evolved into a world of their own to survive in herds that had to feed on available means and then learn how to gather useful food sources, while being able to protect themselves from predators, then monsters and finally hostile ponies as intelligent creatures when they only have hooves, they must therefore have sharp teeth as part of their tooth row. But Sombra was a bit extreme, as these teeth are barely visible and only seemed to appear in certain situations when you instinctively show these to your opponents.
 
The centaurs were probably foragers rather than predators or herbivores by having arms with fingers that enable them to find food sources even in the most barren landscapes, as one can pick out fruits, berries, roots, food plants, small animals and cooperate to take down larger animals just like humans in ancient times. But by having a large digestive system like a horse rather than a pony, it meant that they must have slightly different food sources as they must spend the whole day to gather enough food. Perhaps the Nether Lands and other countries where the centaurs travel have peculiar food sources that we do not know about. It is known at least that the upper body of a centaur is strongly muscular to a much greater degree than of a human or a gorilla, and that they seemed to have double set of lungs which thus makes them more energetic. There must be a logic behind this; perhaps the centaurs must be very strong because they have a very rigid and brutal hierarchy just as with “our” centaurs we know were very violent and aggressive among themselves as well.
 
The changelings eat solid food, this is seen in the show. In the episode “To Change a Changeling” it is seen that these eat large insects that can be cut and torn into small pieces as soon as it is ingested in the mouth. Chrysalis in “Frenemies” can chew like a regular pony, when she ate a cupcake, but it was only a piece of the cup on the cake. Maybe she just tastes this on the back of her mouth where her cheeks can widen out like that of a squirrel. But on the other hand, her row of teeth is a little different from what is observed on all other changelings.

@Background Pony #413A  
Oh, I do intend to draw more at some point, perhaps include more different species like hippogriffs, griffons, satyrs (Storm kind was mentioned to be a satyr-like being, so… maybe? I’d imagine goat legs and howler monkey upper body)
 
Also, horses and ponies are opportunistic omnivores in real life. They can ingest meat, and sometimes do if they are deficient and get an opportunity to do so. So, ponies having baking stuff makes biological sense.
 
And yes, King Sombra’s teeth may be completely natural, if a little unusually grown: normally wolf teeth are not at all that big, and are often removed because they can interfere with a bit. But as ponies here don’t really wear bits, they likely were never removed since they didn’t seem to bother him.
 
Honestly, it is hard to say what centaurs and gargoyles really ate, since their homeland is shown in the comics to be so… barren of life. It’s almost frustratingly so.
 
Changelings can probably ingest food that requires very little chewing, things that you would rip into tiny pieces (I’d imagine the mandibles can move even if the mouth is closed, technically working like scissors when they do get something to eat). However, I don’t think they gain much sustenance from solids.

So the tooth row for King Sombra can be natural instead of an outcome of magical distortion on the pony’s body? That may be right; no matter how strong the background magic may be, it will not avoid following the laws of nature for the least possible deviations and thus can “save” on the intervention itself. We have seen many times that the ponies have visible canines that only appeared as expressions of aggression and anger, it is my head canon that only the stallions and the alicorns have these, which are “hidden away” in the tooth row. But the mares may also have these. It is unlikely that the ponies - especially the pegasi - are only vegetarian, we know they consume animal foods that are necessary for bakery, among other things.
 
As for the centaurs, using the hominid model avoids the ban on the use of humans as the starting point for intelligent creatures on the Equuis. Both Vorak and Tirek have protruding and distinct jaws on them - which seemed to get bigger with age. Thus, one can have a certain resemblance to the equine animal, yet is actually of primate origin. The young Tirek was more human-like than many centuries later. This is hardly surprising, the cenaturs need more food that can also be somewhat varied, among other things to survive in barren countries. We have seen other primal-like creatures, primarily the gargoyles that appeared to coexist with the centaurs.
 
It is hardly surprising that the sirens were predators, as their size and dynamic body shape suggest that they were high up in the animal hierarchy, especially by traveling on the high seas where only other animals are the only food source. Then these teeth are meant to catch fast fish without wasting energy.
 
When it comes to the changelings, it is not so surprising that the teeth have more functions than just chewing and processing food, we have seen that they also eat solid form - meat-based food - as the ability to suck out emotional essences such as love is linked to the internal magic in the changeling body. One of the methods of catching their prey is to simply bite to inject poison so the prey will lose the ability to defend itself. Also, we know that changelings had high aggression towards others as well as towards each other, so their use of teeth will be evolutionary as a tool in several areas.
 
I hope you will find out more about others, including the griffons who seemed to have both beak and tooth row, and that the beak is of a pliable material. But it is in a world where magic can allow extensive body changes as seen with the changelings.

@ABronyAccount  
A very good question. Very good.
 
I think it can be left to preference, but I could see Sirens, as they should still need to eat (negative emotions was said to enhance their power, not feed them like with changelings) and that would likely be fish, mostly. I could very well see them having pharyngeal jaws.
 
Thank you for bringing this to my attention, it slipped my mind. I admit, fish anatomy is not my forte.

Do sirens have pharyngeal teeth? Maybe even … pharyngeal jaws?

@FanOfMostEverything  
Thank you. I also find the idea wonderfully bizarre.
 
I think much like centaurs, gargoyles (not included here) like Scorpan, have teeth more like baboons. Which are fairly similar to gorilla.-

I hadn’t thought about other primates having baby teeth. The idea of a young Tirek putting his under a pillow for the Tooth Breezie is delightfully bizarre.
 
And all the yes on sirens ripping and tearing.