Temos descrições detalhadas de todos os três no cânon. Eles parecem semelhantes na superfície, mas eles não parecem ter muito em comum (além de uma propensão para seres humanos irritantes).
Ghouls
De bestas fantásticas e onde encontrá-las :
The Ghoul, though ugly, is not a particularly dangerous creature. It resembles a somewhat slimy, buck-toothed ogre and generally resides in attics or barns belonging to wizards, where it eats spiders and moths. It moans and occasionally throws objects around, but it is essentially simple-minded and will, at worst, growl alarmingly at anyone who stumbles across it.
Fantasmas
Da entrada de Pottermore para fantasmas :
In the world of Harry Potter, a ghost is the transparent, three-dimensional imprint of a deceased witch or wizard, which continues to exist in the mortal world. Muggles cannot come back as ghosts, and the wisest witches and wizards choose not to. It is those with ‘unfinished business’, whether in the form of fear, guilt, regrets or overt attachment to the material world who refuse to move on to the next dimension.
Poltergeists
Da entrada de Pottermore para o Peeves :
The name ‘poltergeist’ is German in origin, and roughly translates as ‘noisy ghost’, although it is not, strictly speaking, a ghost at all. The poltergeist is an invisible entity that moves objects, slams doors and creates other audible, kinetic disturbances.