Wishbone
Paladin Radmaster
That's what VSHA* is for.
*Villainous Safety and Health Administration
Flippant answer aside, considering the sheer assortment of monsters and beings with varied methods of locomotion, it's no great stretch to have a dungeon be accessible. The last thing the BBEG wants is for their pet wyvern to be defeated by the brave heroes retreating to an area without a wide enough width for its wingspan, or the naga by a set of steps.
Hell, I'm surprised there aren't more prominent options to treat an ooze as a mount given the tone of Acquisitions Incorporated, which I think would be an awesome option.
The DND Beyond video makes some solid points.