THWOMP! and Streeeeeeeetch
Thwomp is a big ol’ spiky rock that will try and crush the Mario Brothers when they attempt to pass the stone, in various Koopa castles and fortresses.
Stretch only appeared in Super Mario Bros. 3, and was mostly just a Boo ghost stuck in a block. They weren’t terribly threatening, except that they could appear on the top or bottom of their block, and sometimes both at the same time. Not terribly threatening, but the Marios still need to beware!