Mountain lions. Ain't even drawn on one yet, much less treed one, but it's had my heart rate up in the "stroke zone" several times.
No dogs allowed for hunting cats here, so I cut trail in fresh snow early in the morning by driving along backroads. When I find trail, I park, grab a backpack with a serious load of survival gear, my bow, and three broadheaded arrows and start chasing along the track as fast as I can go. The idea is to pressure the cat into running, getting mad, and treeing up. Once treed, you can stop long enough to string the bow, nock an arrow and take a shot. Until then, there is no pausing for breath or resting, pushpushpush. I used to try to run 'em with a strung bow, broke a bow and several bowstrings.
I've given up on two cats and lost trails on a few more. It's really tough when you are at the point that you gotta break it off...cuz now you gotta find your way back to your vehicle. AND there is no adrenaline kick to help you back!
That and hunting cottontails, never get tired of that!