Why does my 3 year old keep telling me that frogs are chasing and choking him in the middle of the night?

My son just turned 3. For the past few months he has been waking in the middle of the night saying that the froggies are going to get him and that they keep chasing and choking him. A little violent for a 3-year-old huh?? Is he having nightmares? Is this symbolic for something? He wakes up almost every night. We have tried nightlights but nothing seems to work. Any advice??