User blog comment:Plexioth- xD/Monster Hunter 3 Ultimate/@comment-178.115.27.48-20111231133724/@comment-70.54.113.166-20111231194140

I haven't been able to find anything about Amatsu in 3G aside from a Japanese wiki page (which doesn't include Amatsu in its 3G monsters list). But I think it's plausible. There's probably a true final boss later on.