Players can polymorph dungeon features by zapping a polymorph beam down at it, with some special cases:

• Gravestones can be created by polymorph and are more likely than other outcomes, but cannot be polymorphed into anything else. Trying to do so only polymorphs its epitaph.
• Polymorphing an altar angers that altar’s god and any attendant priest since you effectively destroyed their altar
• Fountains either cannot polymorph into anything besides headstones or have a very limited chance of becoming something else; this is to prevent the player from being able to generate a bunch of thrones from fountains.