Looks like a very young eastern box turtle (Terrapene carolina carolina) who was chewed on by a dog on the back end. It's old injuries and all healed up now. It looks more to me as if there were some either dried on poop or dirt around the tail, but its really hard to tell in a picture. You are seeing the turtle in real life. So your guess is better than ours. I'd like to say no worries, but I can't really tell in a picture. What a precious face. How did this turtle come to find you?
(Incidentally, if you ever need to re-home this turtle, I'm your gal! I LOVE the "special" box turtles)