Ghouls

Ghoul Where does the word ghouls come from? Before it came to be applied to any random sicko or serial killer it meant went all the way back to the medieval Arabic ghul...it will dig up graves to eat the dead. – Alex Irvine, The Supernatural Book of Monsters, Spirits, Demons and Ghouls

In 4.19 Jump The Shark, the offspring of a ghoul killed by John Winchester lure Sam and Dean to their town in order to take revenge. They can shapeshift by taking the form of the last person they ate. One takes on the form of Adam Milligan.

Ghouls are spirits of the dead that traditionaly feed on the blood of the dead (the sole exception of the two ghouls in Jump the Shark)and can only be killed by decapiation or the destruction of their head.

In some lore they are not dissimilar to zombies, being corpses reanimated by witches or demons that can be commanded.