cromat
History is usually written from a subjective perspective with the hopes of writing objectively. For better or worse it's easily possible that it's written and known as millions of years simply because they've existed ever since history has been written.
personally i think that when it first started it was just supposed to be a gag manga so the author didn't put much thought into the lore of creepy cats. but then she decided to make this arc and so rewrote the whole backstory of the creepies
even recently (intro chapters of flora's friends) saw the creepy cats that evolved into seals on that beach implying that they've always been there and never went back.
tho this is probably gonna end in a fakeout where they are only gone temporarily.
@noonehere_shoe It's possible that witches were the ones who brought the Creepies over long ago to use as slaves leading to Creepies hating witches on instinct. Also Red hair could be an outlyer and want to go against other witches and send the Creepies back, she probably killed a lot other witches to take back that many cats.