How Now Brown Cow?

So... for a while now, Aidan has recurrent nightmares about cows. Yes, cows. I'm not sure when, exactly, the terror of bovines started (and, BTW, he's not scared of cows when he's awake- go figure!) but it's a real source of fear for him in the middle of the night. A couple of months ago, Wayne and I were watching TV and Aidan came stumbling down the hall, mumbling something about there being a cow in his room. Wayne took him back to bed and, not 2 minutes later, Aidan came running out his room, looking over his shoulder, yelling that the cow was back. Since then, he, occasionally, talks about cows in his room. And, no, he does not have any cow pictures hanging on the walls, cow toys, etc. that could, indeed, mean a cow is IN his room. Any way, this past Monday, he woke us up again by crawling into bed with us in the middle of the night. The next morning, I asked him why he got out of his bed and he told me that there was a cow in his room again. Before he went to bed last night, I told him to have Daddy tell the cows to stay out. Lo and behold, he stayed in his own bed the whole night! Out of nowhere this morning, he told me "Guess what, Mom? The cow didn't come into my room last night!" I guess we're going to have to start verbally banishing imaginary cows every night before bedtime....