List of exclusive raids and when they were available/expected to released?

Any one have links to any of exclusive raids and when they were available or set to be made availavle? I took a minor break from the game and came back around the time MewTwo was in raids, so I'm curious what else has been out there?.

ex: I saw an Absol today, but no one was around bk to help catch it. How often does Absol appear in raids? Has it been out for awhile? Thanks in advance