Although I voted (snakes) it's not so much that I "fear" them - I have a fairly reasonable concern for my kid's safety since a few are poisonous enough to do reasonable damage to a kid. The reason I selected them is that they have a tendency to lay VERY still either IN or NEAR enough to hiking paths that you can be on top of them before you know it (literally - stepping on one followed by the snake's retaliation is reasonable).
If I ever camped anywhere in Bear country - that's likely what I'd vote for though. I've got a pretty healthy respect for any animal that's willing to (and capable of) rip the door off a car to get a cracker.
My son however is freaked out by hornets. Specifically the ones that show up at a campground we've been to a couple times. Course, they are about an inch and a half long with retractable stingers that look to be about 1/4 inch long.
When the big kids were younger - they'd panic when they saw a horsefly.
When my daughter was much younger she'd run screaming away from "Spiders so small you couldn't see them".
mkw