Well You know, that's nothing sure, there never was shown pony, who eat some meat(and biological inability of digest meat is quite likely, considering that ponies - even if magical, colorful, sentient and all this stuff - are surely quite related to ordinary ponies or horses, which are herbivore. But nothing sure was told).

On the other hand(or hoof) let's consider this:

Animal on farms are usually kept for some purposes. Chicken mainly for eggs, cows mainly for milk, this both also sometimes for meat. And pigs, pigs are usually kept for meat.

What animal there are on Applejack's farm? Chicken, cows and pigs. Two first are easy, and make no problem to herbivore theory - ponies eat eggs and drink milk(and also used them for cakes etc.) - but pigs? What's with pigs? Keeping them for no reasons seems not likely. Especially that we add som other fact to this. We saw cows are sentient in MLP, but pigs doesn't seem to be so(chickens too, but as i said they can be keept for eggs). Of course that's not a ultimate prove, but it is some hint. What more in one scene we could see something whats look like ham slice...