ومسنونة زرق كأنياب أغوال
And sharp, bluish points like the teeth of ghouls.
The ghouls (aghwāl) belong to frightening imaginative lore; their teeth are not established as externally observed things. Nonetheless, the mind grasps the sensible form of teeth and attributes them to the imagined creature.
This is called wahmī: a form is supposed that, if it existed externally, could be sensed. The comparison does not establish the creature’s existence; it conveys sharpness and terror through a hypothetical image.