There’s all kinds of silly names they could borrow from stories hypothesizing its existence. Nemesis, Nibiru, Vulcan, and Planet X, just for starters. But in all likelihood it will end up as something much more anodyne.
There's a region on Pluto provisionally named after Cthulhu, citing Lovecraft as "an underworld mythology". If the IAU accepts the name, it would make this possible too.
https://lovecraft.fandom.com/wiki/Yuggoth