Shadowbeak no longer available to breed?

Sorry if this has been answered already but, I've tried the breeding combination of Astegon and Kitsune which resulted in getting Relaxaurus' on my quest to get a Shadowbeak with the Legend passive. So when I went to the Google and found out that Shadowbeaks can only breed themselves now, i though perfect! I will just capture a couple shadowbeaks. Done and done, and then BANG! More Relaxaurus. Back to Google to find something about one of them needs to be a tower boss and one regular, only to find out you can't capture a tower boss anymore! Can someone please inform me of what he'll im doing wrong...