If you read Beowulf, you will see that Grendel is a vicious monster who doesn't care about humanity at all - people are there just for him to eat. On the other hand, if you read Grendel, you will see what happened from his point of view - that he is not just a brainless beast, and that he has thoughts and feelings.
The Monks Tale fits the category of a parable, because a parable is a story that is used to explain something else, which is what the monks tale is doing.
