Vorpal bunny, a magenta r monster with a deadly bite: it decapitates you if your HP is at 50% or less, and bisects you otherwise. Probably a reference to the killer rabbit from Monty Python and the Holy Grail.

It should probably only generate in Gehennom, or be unique and generate in a specific area or off-branch of Gehennom, or be psuedo-unique by giving it a birth limit of 1.