You can't trade them back even though they are in the same Gen. You can't because they weren't in the game and they weren't even coded in those games. Source: https://www.reddit.com/r/PokeMoonSun/comments/7jsgy1/cant_trade_blacephalon_or_poipole_back_to_sunmoon/?utm_source=BD&utm_medium=Search&utm_name=Bing&utm_content=PSR1
