The sandestin, &, is the rarest seen of the shapeshifting monsters in NetHack, only being generated in Gehennom. Despite having the glyph of a major demon it is not considered as such. Sandestins have a 1/6 chance of changing form per turn, and will mimic nasties 6/7 of the time, or a random monster the rest of the time. It is quite possible to play nethack for a long time without knowing the existence of these monsters, as they always mimic other monsters and leave no corpse.
Ildefonse left the terrace and almost immediately sounds
of contention came from the direction of the work-room.
Ildefonse presently returned to the terrace, followed by
Osherl and a second sandestin using the guise of a gaunt blue
bird-like creature, some six feet in height.
Ildefonse spoke in scathing tones: "Behold these two
creatures! They can roam the chronoplex as easily as you
or I can walk around the table; yet neither has the wit to
announce his presence upon arrival. I found Osherl asleep
in his fulgurite and Sarsem perched in the rafters."
"No matter," said Rhialto. "He has brought Sarsem, and this
was his requirement. In the main, Osherl, you have done well!"
"And my indenture point?"
"Much depends upon Sarsem's testimony. Sarsem, will you sit?"
"In this guise, I find it more convenient to stand."
"Then why not alter to human form and join us in comfort at
"That is a good idea." Sarsem became a naked young epicene
in an integument of lavender scales with puffs of purple hair
like pom-poms growing down his back. He seated himself at
the table but declined refreshment. "This human semblance,
though typical, is after all, only a guise. If I were to put
such things inside myself, I might well become uneasy."