I'm pretty sure the goat WAS the devil. It's been awhile since I've seen the movie, but doesn't the goat speak at the end to Thomasin before suddenly he's in human form as the devil himself? A possession could be the same thing, but I remember having the impression that the goat was the devil the whole time, and when he appeared as a human at the end, the goat just wasn't there anymore.

I assumed it was possession, and likely occurred right before we first see Black Philip which has escaped and is dancing/kicking in the yard with the little children. This is also the first time we see (or hear of) the father fighting with the goat -- good vs evil. Didn't they have the goat for awhile? If it were the devil incarnate then why didn't it simply do what it did before?

@Dedoc1967 said:

I assumed it was possession, and likely occurred right before we first see Black Philip which has escaped and is dancing/kicking in the yard with the little children. This is also the first time we see (or hear of) the father fighting with the goat -- good vs evil. Didn't they have the goat for awhile? If it were the devil incarnate then why didn't it simply do what it did before?

Perhaps the devil possessed the goat at the plantation and prompted the smaller childrens' incessant singing or just flat-out kept escaping its pen. In a hyper-paranoid society those factors could have been the grounds for banishment. All part of Satan's plan to get them out into the woods...