I have holes all over my yard, they are not moles because there is no dirt pushed up. I live in southern Colorado

How large are the holes? If they are small it can be from insects. If they are large then it can come from larger animals like groundhogs.

It still could be moles. I had moles in my yard and there was no evidence of pushed up dirt.

Chipmonks, ground hogs, snakes ?

Ha... I just realized you said Southern Colorado... it is possible that these holes are from tarantulas... Aphonopelma live in Southern Colorado and they do burrow. Their holes are about the size of a quarter up to about a 50 cent piece. They usually have no noticeable dirt mounds around them and quite often don't have visible webbing either. They are pretty much harmless so if it is them, nothing really to worry about.