Slenderman Monster

[Kman]

The body shape of that model is very different from that of the Grunt, so not only would the skeleton (probably) not fit, but if it did and you ran the animations it would look really awkward. If you were to use that, you'd probably have to make an entire new skeleton and set of animations (though I don't see why you'd want it to move like the Grunt considering that most of the time Slenderman is completely still and just teleports around).