I didn't pay close attention, but since TNG several of the Trek shows have had a couple of top-billed actors and then listed everyone else alphabetically. Bruce Horak is listed alphabetically in the episode 2 credits, and possibly in the first ep as well. I noticed recently, in watching the pilot for NCIS: Los Angeles (my partner is a big fan) that Linda Hunt only became regular cast in episode two of the series - she's listed as "Special Guest Star" in the pilot. If Horak was ever just a recurring character, that changed quickly. Same, I suppose, as with Michael Dorn.