Misdreavus is a corruption of the word mischievous. It may also be any combination of the words miscreant (due to the way it always scares people), dream, dread, dreary and reave (to rob). Its Japanese name Muma, is likely to be a corruption of 夢魔 muma, which means a nightmare or demon that features in dream.
Misdreavus is a dark bluish-green ghost. While it has no visible arms or legs, its lower half has several small appendages and resembles a ruffled dress. It has several round, red orbs around its neck, which are used to absorb fear from its victims that is later used as nourishment. It has long, flowing lilac-tipped ‘hair’ and its large eyes have yellow sclera with red irises.
They live in caves and being nocturnal, spend their days sleeping in darkness and at night enjoy startling people with mischievous tricks. Since it loves watching scared people, it is known to yank and bite at people’s hair or sneak up on them and cry or shriek.
Misdreavus’ nocturnal habits of wailing to frighten people and similarity in appearance to a disembodied head are shared by various mythological creatures. One of these is the ‘Nukekubi’ of Japan, which shares its similarities of flying about as a decapitated head and wailing to increase its victims’ fright. It is also analogous to the ‘langsuir’, a type of vampire in Malaysian mythology known to appear as a flying decapitated female head.
PLEASE TAKE A LAMINATED IMAGE OF MISDREAVUS FROM INSIDE THE CACHE AND HOLD ONTO IT. REMEMBER TO KEEP A LOOK OUT FOR FURTHER CACHES IN THIS RANGE, APPEARING IN THE FUTURE. YOU GOTTA CACHE 'EM ALL!!
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TankTracks

Image & information sourced from Bulbapedia (www.bulbagarden.net)
This is official artwork by Ken Sugimori & the contributor (Bulbapedia) claims this to be fair use.