A thought about the "shadow man"
*Possible Spoilers Ahead* (though why you'd be reading the message board if you haven't yet seen the movie is beyond me)
I watched this film last night for the first time in years and I just figured that the man seen only in silhouette early on (who invites the Dog-Thing into the room) was Clark. After all, he was the one who was so fond of dogs, and I thought the silhouette looked like him. It wasn't until reading about the film online that I realized that it's supposed to be ambiguous as to who it was.
Just throwing that out there.