You can’t ride clinging monsters (how would you get onto them, or stay on them?).

Add a level flag for the level being open to the sky or otherwise no ceiling. Effects of this include: falling rock traps do not generate, cursed potions of gain level do not work, scrolls of earth do not work; monsters that normally cling to the ceiling are unable to. Levels that would have this: lots of Quest levels, the Elemental Planes.

When you ride a clinging monster like a wumpus, you get a message about all the blood going to your head.