In the Trap Door episode "Ghoulies", Berk finds a book of magic spells while tidying up. Intrigued, he reads a spell from the book which causes the sudden appearance of a group of ghosts that start turning everything topsy-turvy. Berk reads the spell again backwards in order to banish the ghoulies.
The ghoulies are not made of plasticine like the rest of the creatures in the show but are animated in 2D effects. They appear as wisps of green energy and constantly emit an eerie moan.