Xorn

A xorn is a particular monster that can walk through walls and is stoning resistant. The xorn won't be seen until the player has reached a relatively deep dungeon level. If you polymorph into a xorn, you can eat amulets and metallic rings, sometimes giving you their effects permanently (as intrinsics), and you can also eat your junk weapons.

The xorn is the only member of the xorn monster class.

Encyclopedia entry

A distant cousin of the earth elemental, the xorn has the
ability to shift the cells of its body around in such a way
that it becomes porous to inert material.  This gives it the
ability to pass through any obstacle that might be between it
and its next meal.