Mystery Caterpillar, Maybe Tiger or Tussock moth?
This fluffy guy shows up sporadically throughout late spring and early fall, but I haven't been able to figure out what type of butterfly/moth it will turn into. The closest I've found is that it's either a tiger moth or a Tussock moth; I have seen Tussocks but not tigers flying around. However, aside from one or two photos online, all the Tussock caterpillars I've seen are too pompom-like, with very tall bits of fluff along their back, rather than the even tufts this guy has. None of the online photos have ad the red head spots either--I've seen some with only a little red, and others with a few segments worth. The below photo is really the only good one I have of one.
Larimer County Colorado
It is the larva of a tussock moth of some kind, not sure which one. Most tussock moths are in the genus Orygia http://bugguide.net/node/view/335/bgimage and I think it is one of those.
Euchaetes species are also commonly called tussock moths http://bugguide.net/node/view/243/bgimage as are a few other moths that have caterpillars with tufts of hairs.
The most common tussock moth in many areas of Colorado is the douglas-fir tussock moth which feeds on douglas-fir, true firs, and spruce. It is possibly that one.