Q. I have run across this type of construction frequently in a fiction manuscript I’m editing. It feels somehow wrong, but I can’t find any reason why it should be. “Next to the door stood a single guard, an ugly aardvark that was staring at the ground and didn’t see them approach.” A colon instead of the comma would feel better, but is that an unnecessary change? (And colons look rather formal in fictional narratives.)

A. The sentence is fine. (Grammatically, at least.) The comma shows apposition.