Untitled Sighting

winter of 1980, my older brother and i were hiking and camping in yellowstone park. after a five mile hike from the road up to a lake,(that reportedly was the only place in north america to catch arctic greyling other than in the arctic,) we set up camp for the night. after eating the days catch, and cleaning up, we settled in for bed early as we were tired from the days hike and fishing. we were in our tent, quietly talking and starting to get drowsy when all of a sudden something ran by us- on two legs....something very large and heavy. we looked at each other with eyes bulging out of our heads, and i told him to go see what it was.....he replied,  'no f...... way' and as we had no sidearms or weapons, just sat there terrified for the rest of the night. at daybreak we broke camp and left there looking for tracks but the ground was bare and there was no sign of our midnight visitor except for our experience