The Sims Medieval never really explains the Pit Beast kept at the Judgement Zone - it's just kind of there.
However, while I was playing The Sims 4, a child in my household got assigned a school project to build a Medieval diorama. When it was complete, the flavor text on mouseover specified "Sixam Pit Beast not included".
So TS4 has not only put the Pit Beast in a historical context for modern Sims (implying that TSM is in fact the historic past of that world), but now explains the creature to have originated from the Alien planet, Sixam!
I just like that the Gurus actually curate all this accumulated Sims lore, even from odd side-games like TSM, and continue to build on it, later.