Marmot (aka "Ice Age Guinea Pigs")
An earlier version of this page listed this as a Mountain Lion. I've seen a number of them since (always at night) and I'm quite sure it's Bobcat and not a Mountain Lion. There's no obvious long, powerful tail and he/she appears to be fairly small. There are no spots and the ears appear rounded, but the real tell is the eyes. Bobcats tend to have a white perimeter (as in the picture below) and mountain lions have a very dark perimeter.
'Cow Killer' Ant (Mutilidae)
I'm still working on identifying these. If you know what they are, please let me know.
Common Trees in Colorado: csfs.colostate.edu/colorado-trees/colorados-major-tree-species/