A ghoulie is a little demon. Ghoulies are usually summoned from Hell by black magic spells. Ghoulies come in all shapes. Some appear like animals, some don't. Not all ghoulies are evil. Their world is never seen in the movies. They can't be killed by normal means. Only magic can beat them. In the first two movies, the ghoulies usually don't talk, but in the last two, they do. Ghoulies like Lite and Dark are completely different, which make the others a sort of subspecies.
Here are all of the ghoulies.