Of course it wasn't actually Walt. He didn't just swim back to the island.

I was so excited to see him again. If Micheal is going to be an idiot and never come back to the show that is fine but I want more Walt. They left with so many questions. Anyone else notice how old he looks.

It is the island talking to him again. Not Jacob because it hasn't even been mentioned that Jacob can change his form and be other things. It is the monster that took form of the horse, Jack's Dad, and Walt earlier.