Untitled Sighting

While bird hunting with my son, we heard a rustle. As any bird hunter, the gun came up but I expected (because of sound) to see a deer startled from sleep. Instead, I glimpsed something quickly moving between the trees (about 8' apart), over the ferns, and barely below the lowest treelines. Admittingly, this couldn't have been for more than 3 seconds. This was a very quiet day with very little breeze; it wasn't so much what I saw, but what I heard.  If I was deaf, I could dismiss it as a human trespasser. It was all dark brown, about 7' tall, and running (extremely fast given the original "startle") silently through the ferns, branches, and sparse deadfall. It was not a bear. I was not a believer of bigfoot. I told myself it was a deer leaping (because of the silence and upright position) that covered about 11yrds. Acceptable, right? I looked down, at my son and saw nothing but pure white fear in his face (see below).