This one is pretty common I would imagine, but seeing a northern water snake with a very decent sized rainbow trout in its mouth on Bradley Fork was a sight to see. I heard spashing upstream and wondered what it was. Then it got closer and closer. The fish was putting up a good fight, but eventually the snake was able to crawl into a clump of branches on the bank to enjoy its meal.
On Slough Creek in YNP, I witnessed three coyotes take down a fawn. Lots of commotion upstream as they ran through the river, then across. The mother doe was too late. She tried to fight the coyotes off, but the kill had been made. The doe stayed by her fallen fawn the remainder of the time I fished. Came back the next day and she was still there.
Had a staredown with a moose on the Firehole in the Fountain Flats area. That wasn't much fun.