A lot of times those big cat sightings and black panther sightings prove to be big domestic cats gone feral. There are different breeds of domestic cats that color out differently, have different tails, grow to different sizes so I wouldn't go too much by that. That looks a little stocky to be any wild cat I'm familiar with, though sometimes exotic pets get loose in the wild.
I think I have a good feel for the size of that rock, I have one about that shape that meets your description. I think the second shot gives a good perspective of the size. The first one may be a bit out of scale but that second one shows that is a big cat, whatever kind it is.
I'm not totally sure it's a cat either, the shape doesn't look exactly right.
