Once when I was following my wife on a descent at probably 25-30mph, I saw a squirrel dart out into the road, nick her rear wheel, kind of "bounce" in the air, and run off. My wife managed to stay upright; it could have been much worse.

This happened the year I did Levi's GranFondo. I didn't see it in person, but heard about it afterwards (not a pleasant pic).

Cyclist recalls being 'torpedoed? by squirrel - SFGate