Is it safe for my dog to eat raw fruits and vegetables he steals?

It could be that you aren't feeding Marley enough, and he's going out to get more food to feel full. It could also be that he's tried them for the first time, and likes the taste of them. While most of these are fine for him to eat, there is concern about him eating the apples and peaches. Apple seeds contain small amounts of cyanide, which when too many of them are eaten can lead to toxicity. The peach has a very large pit that if eaten, will cause an intestinal obstruction. The pit also have cyanide in them. I would keep him from eating the horse's food, and make sure you are feeding him according to the package instructions on his food bag. I hope this helps!