I love Mystara and the 2E bestiary, but I sure hope WotC doesn't feel compelled to go dumpster diving for often justly forgotten monsters from over the years. It was great that they brought forward magen, and I'd love to see them make the jump to the 2024 Monster Manual, like the nagpa did in 2014, but most of the obscure BD&D weirdos can and should stay forgotten.

EDIT: Nagpa were in Mordenkainen's Tome of Foes. They blur together for me, since I came back to D&D after it and Volo's were already out.

EDIT 2: Also, the nagpa should make the jump to the 2024 Monster Manual.