I logged in to my Singleplayer world to find a broken skeleton mob. I cannot kill him or damage him, he does not attack or move, and arrows shot at him bounce off the entity. Newly spawned skeletons seem to act normally. He still pushes me away when I walk into him, but he still doesn't move. The only thing that works is the idle skeleton animation.

What was supposed to happen is pretty self explanatory.
see attachment of screenshots

I'm not sure how this happened, he spawned in while I was playing and I noticed him standing there.