They use pigs for manure

Also, pigs seem kinda dumb in MLP. It could be that they use them for leather.

they sell pigs to the gryphons

@Background Pony #479A  
Teachers generally don’t knock around kindergarden age kids in their care. I hear the parents tend to disapprove of such teaching methods….

@Background Pony #84BA
 
Like herding kindergarteners for school. They may not want to be there but it would be wrong to call them SLAVES.
 
Also, wool, obviously. And if those sheep wanted to, they could jump over the fence easily.

thse pigs are clearly garbage disposals

@Cyborg_pony  
noted

@Delzepp  
Because Soy ham is for heretics!

@Background Pony #84BA  
or soy ham
 
why can’t it be soy ham?

@Ebalosus  
That’s clearly just some kind of pink vegetable in that sandwich. Clearly.

*Ahem*
 

I think they keep them around for Truffle Hunting and waste recycling.  

@Thegmaster  
So, Apple Bloom rides on Big Mac’s shoulders, and they are trying to wrest control of the farm from Auntie Applejack because they control the pigs and therefore the methane. But Auntie Applejack has figured out a way to get a drifter named Max to put things back under her control. Two go in. One comes out.

@Thegmaster  
Those things are powered by unicorn magic, like all non-manually-powered equipment in Equestria.

wrong, pigs mean that she has a methane farm for producing that natural gas which she uses to power her farms such as sprinkler systems or milking machines.

@SolarHedonist
 
IIRC a few fanfics have Pegasi cloud cities established over the ocean having fish as a major part of their diets simply because it’s not practical to produce for a vegetarian diet so far away from the coast. Some other fanfics have Pegasi in general being more comfortable with meat eating than unicorns and earth ponies due to interacting with griffins more often. It’s fanon, but both make sense.
 
The series itself shows that ponies are for the most part are ovo-lacto vegetarians though.
 
 
@Zergbrony
 
True, probably low scale organic waste disposal is why they have them. They don’t really have many pigs so fertilizer for the orchard is unlikely. I’d guess if anything it’s being used for a smaller scale garden or being sold to gardeners in Ponyville. The orchard is huge and it looks like they have less than a dozen hogs. They’d need far more than that for fertilizing the orchards.
 
They’re probably getting fertilizer shipped in from somewhere else or possibly getting it from the cows on their farm.
 
The cows though since they are sapient, definitely interact with ponies a lot and eat the same food sources and such. Using cow manure in this setting is probably a bad idea since it increases the risk of disease transmission. There’s a reason developed countries don’t use human waste as fertilizer, though it’s been done in the past. It’s a disease vector. The same would apply for ponies and any sapient species that commonly shares space with them.

Ponies/horses can eat meat though. It’s common to feed them meat in extremely high altitudes or extremely cold climates. There are even a few documented cases of predatory, meat eating equines in the wild although it’s very rare to see. By the real world physiology of equines at least, pegasi probably do include meat in their diets since they fly high and it’s very cold up there.

@RealityCheck  
They do eat hay though.
 
Season 1 episode ‘Bridle Gossip’

@Somber Star  
I think the reason they don’t eat grass is because of social stigma. As in “Only the truly, desperately poor eat something as dirty, tasteless and unrefined as wild grass.”

sigh
 
Truffles.  
Garbage disposal.  
Fertilizer.  
Methane (if you’re green-revolution minded.)

@Taupe_Hat  
pigs are quite useful in disposing of waste organic matter and turning it into fertilizer, two reasons to keep the pigs around not related to food

@Background Pony #3B4A  
They tried once:  

The CMC are pigs?

The ponies, like actual horses, are herbivorous. Unlike actual horses, they can eat things like daisies and human glucose-based products like cakes and such without severe health problems. They also don’t eat actual grass, which is odd, because in Equestria the grass is lush and abundant. This in turn creates a dependency on Earth ponies, who are the only ones capable of providing food.
 
As far as disturbing fridge logic goes, I still think the worst are the facts that slavery and wars are still things, although if Over a Barrel and the Canterlot Wedding episodes are any indication, the wars might in fact be short and possibly even comical.

@furbyguy  
All that being said, I’m still gonna pretend they’re omnivores.