With some kids, it's monsters.
For Jonah, it's "bad guys."
These "bad guys" are just as nebulous and undefined as "monsters," however.
He makes this statement (not a question, a statement-- presumably for his own reassurance more than anything) several times each day: "There's not any bad guys."
Jonah: "Daddy, there's not any bad guys, okay?"
Daddy: "Well, there are bad guys, but God protects us from them."
J: "No! There's not ANY!"
D: "Okay, there aren't any in our house, and there aren't any in our yard, and there probably aren't any bad guys in the neighborhood, but there are bad guys some places. You don't have to be afraid because God will keep you safe."
J: "No! They're all in jail!"
When I tell Jonah to go upstairs and get some socks, he pauses, looks worried, and then says, "There's not any bad guys."
"No bad guys upstairs. Go get some socks."
When I tell him to take the compost bucket out, he says, "Not any bad guys." as if to steel himself for the journey to the compost pile.
Explanations of protecting angels helps, but sometimes, when he talks to me about it (especially at bedtime), I don't even bother with explanations of why he doesn't need to be afraid. I just agree, "No bad guys." He can be a little 'fraidy cat sometimes, and I figure he just doesn't need one more uncertain thing to worry about. He's becoming more aware of his surroundings all the time and he's learning that the world is a scary place. He'll have plenty of time to learn about the "bad guys" when he's more emotionally equipped to understand. For now, I just agree-- "No bad guys."