Ghouls were a very traditional and classic type of Magical Creature in the Wizarding World, found in the settlements of many wizards and even Muggles.
Traditional ghouls—such as European ghouls and Arabian ghouls—had faces like wild dogs, enormous fangs, gaunt bodies, and a rank stench. Some powerful ghouls also possessed simple shapeshifting abilities, allowing them to turn into hyenas or similar creatures.
Though they looked evil, they were not particularly dangerous Magical Creatures.
This was easy to understand from another perspective. If they were truly dangerous, they would not have been reduced to surviving on the corpses or even rotten flesh of other creatures. Their terror came more from their death-loving habits, born of long contact with death, which had saturated them with the aura of death and given them their horrifying appearance.
Ghouls were not very intelligent. They lived in groups and liked hiding in dark, damp corners. If they could not find corpses, they would hunt spiders, moths, and frogs for food. Old-fashioned wizards often kept ghouls near their homes to frighten away Muggles who wandered too close.
Unlike traditional ghouls, however, the ghouls of the Illusory Dream Realm—though likewise nocturnal humanoids that fed on carrion—had evolved into a more advanced kind of Magical Creature due to the existence of Mordiggian, the King of Ghouls, and Nodens, Lord of the Abyss. They had religious organizations, could use a gibbering language made up of simple words, travel freely between the Illusory Dream Realm and graveyards in the real world, and even cooperate with night-gaunts to form armies and carry out complex tactics.
These subtle yet obvious changes made the ghouls of the Illusory Dream Realm seem more like "people."
More importantly, ample research evidence showed that through certain evil Magic Rituals, Muggles and ordinary wizards alike could be transformed