Thoughts:
We have a lot of articles where SCP-XXX turns its subjects into SCP-XXX-2. This one takes it a step further by adding the completely useless stage where the affected equines become SCP-XXX-2 before becoming SCP-XXX-3. Since there'd be no reason to have special containment procedures for SCP-XXX-2 or -3, as both of them are the same as normal horses and die really quickly anyways, I think you can do away with them and just refer to them as subjects or animals or whatever.
Also, I'm not excited with the idea of the object itself being a feed bag. I mean, I guess it's kind of related, but it seems like a house or something else would be more interesting (granted, it's not like the horse would be talking much, but as soon as it sticks its head in the window one day while the family's eating, BOOM, talking horse.)
Also a picture, of whatever the object ends up being (probably not one of the horses being people) would be good.
I think this is the kind of thing that could benefit from a selection of an experiment log (though not necessarily a whole separate log)- since horses are such gawky animals when you think of them doing things people do, a few gruesome examples could make it much more interesting.
Overall: I don't think the idea's terrible, but the article could use some improvement.