I always forget about the Ultra Beasts as a concept because I'm not super familiar with Gen 7. When I saw Guzzlord in Showdown, I thought it was a fakemon because it looked so out of place. I forgot Phione exists as a Pokemon as all I can picture is Manaphy. Anything Gen 6 onwards I can struggle with as I am less familiar with the games. I can't even think of Pokemon I've forgotten from these generations as most of the Pokemon are drawing blanks in my head. My mind totally blanks when it comes to SWSH pokemon because I have yet to play the games and for the life of me cannot remember any new Pokemon unless they add to the form of an existing Pokemon like Galarian Weezing. :D