Personally, I see the alleged vegetarianism as more of cultural taboos than actual diet.

Beef? Cow and Minotaurs are sentient

Bird? Griffons

Pork? not sure if Pigs are sentient

Lamb? See the second line except for sheep.

the only thing I can see ponies not having a issue for is fish as there is noone to offend by eating any.

That and we've seen ponies fish

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Well we have seen Fluttershy okay with feeding worms to birds. They knew what gravy is. Got to have meat to have gravy.

And marshmallow has gelatin which is an animal product.

I'm pretty sure there are vegetarian recipes for marshmellows and gravy. A quick google search has vegan recipes for the two and I think the vegan gravy uses soy sauce for flavoring while the marshmellows use agar as a substitute for gelatin.

But yeah, I'm pretty sure their vegetarianism is more about the cultural taboos than simply diet.

Though I'm pretty sure eating parasprites would be totally okay, especially after the pests ruin a few crops. I know locusts are okay to eat in many cultures.

I can imagine the parasprites being a problem until the ponies use the poka music to lure them away but then they have to decide what to do with them. They could start by feeding them to the various predatory animals, a passing griffon might want some meat and cooks up some, and soon there is a recipe for fried parasprite bits or something.

I can imagine them being vaguely like shrimp or something depending on how they are cooked.

I posted this in a comment on one of my own stories:

Real world horses can eat meat for extended periods of time. Some hyper-aggressive horses have even been known to eat humans. (Poor Lyra...)

Since plant matter is harder to digest than meat, strict herbivores have a larger intestinal length than a strict carnivore of the same size. Compare the human (more carnivorous) abdomen to the abdomen of a great ape (more herbivorous). Even a healthy human would look like they had a pot-belly if they had an ape's intestines. Anyway, my point is that an herbivore has far more time to absorb nutrients from each meal than a carnivore does. (Not that it would help much. There has to be a balance between amount taken in and the energy you expend to take it in.) You can get 100% of the nutrients you need if you eat the entire animal.

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You mean sapient.
All animals are sentient it is just humans that are sapient.

My head canon:

In the pre-classical era, the Great Windigo Winter fell across the land. Some fled the land for the warmer south. Some stayed. Two new tribes were formed by those who chose to stay in this frigid land, with two very different ways of adapting. One created magical crystals to warm small pockets of land, powered by love, these became the Crystal Ponies. The other tribe survived by eating the Crystal ponies. This is why the Crystal ponies have so many cutie marks that are weapons and fortifications, and why they seem so militaristic (jousting, the word "Empire" rather than kingdom), they had to keep the barbarians at bay.

Though thought to be extinct, some Earth ponies are descended from these carnivorous ponies. Most hide this fact and live quiet lives, separated from large settlements (like on, for example, rock farms), living in peace and just wanting to be left alone. But occasionally they find the urge to consume meat overwhelming, especially if they, for whatever reason, lack strong impulse control