Geese!!!!
I've seen a few threads about the danger of squirels and deer, but my biggest bike path foe has to be the Canada Goose. Our city is lousy with them. And, unlike other critters, they rarely get out of the way. They're normally in a group, and on occasion they attack. On a busy bike path, this means having to completely stop on occasion and wait for the giant feathered rats to move. I have yet to hit one straight on, but I've clipped a few as they scurry at the last moment and take off in front of me.
Am I the only one that HATES these animals?