Pffffft.......for me it's always dogs.  Either behind a gate but scaring the shot outta me or unleashed by their owner who just swears they are never gonna bite......running after me.  I've yet to be bit but those are the "buds" i meet on my runs.

There are some dirt roads that are great to run but I won't run them anymore because of dogs.  I've read that dogs will always back down if you're authoritative enough, and that has been my experience, but a couple unleashed 150 lb dogs baring their teeth and snarling at me is enough to make me avoid certain roads.

I don't mind snakes if I see them in time.  I had one close call with a copperhead that I didn't see until it struck at me and a couple times in Oklahoma I had very aggressive bullsnakes chase me off a trail, but those are the only negative experiences.

In Sri Lanka, I was running ahead of my wife when a cobra crossed in front of her.  I was bummed that I missed it.