Maurice Casey makes some interesting observations on the story of the Gadarene Demoniac- which I quote in full-
The first aspect of the story that is untypical of Jesus, but widespread in stories of exorcism, is that, even after making an effort to order the unclean spirit out of the man (Mk 5.8), Jesus has to ask it its name (Mk 5.9). This is narratively convenient so that the storyteller can tell us its name is ‘Legion, for we are many’, the first indication that the storyteller was disenchanted with Roman legions.
The second feature untypical of Jesus, but widespread in stories of exorcism, is that Jesus sends the demons out in such a way that they visibly enter something else, so they can be seen to have gone out. What they are sent into is a ‘large herd of pigs’; indeed somewhat belatedly the storyteller entertains us with the information that there were about 2,000 of them! (Mk 5.11-13). Pigs were notoriously unclean animals, because Gentiles kept them and ate pork, as Jews did not. From a Jewish perspective, therefore, pigs were especially suitable animals for unclean spirits to be sent into. The existence of a herd of 2,000 pigs, though not strictly miraculous, is not something that would ever happen in real life; it is part of a story told to entertain people, and enable them to marvel at Jesus’ ability to defeat the powers of evil with the power of God.
At this point, we can be more precise about the ‘Legion’. The author had in mind the tenth legion, Legio Decem Fretensis, which had a boar as one of its symbols. It was stationed in the province of Syria, firstly at Cyrrhus, so it was the northernmost of the Syrian legions, and then from 18 CE onwards in the client kingdom of Commagene, which was annexed to Syria. The otherwise powerless storyteller has made great fun of a legion. The effect of Jesus sending the demons into 2,000 pigs is equally entertaining: ‘the herd rushed down the steep bank into the sea . . . and drowned in the sea’ (Mk 5.13). This effectively gets the demons back into the underworld where they belong, for the story assumes they go down to the Abyss. It also dumps a legion where many Jews would have loved to see the Roman legions go.
But the storyteller, a Jewish Christian entertaining Christians miles away, where he knew about Decem Fretensis, was regrettably unconcerned about the geography of the Decapolis. Whether this took place in the country of the Gerasenes (the original text of Mark) or the Gadarenes (some manuscripts which were influenced by Matthew) is the difference between whether the pigs had to run 33 miles, or just 6 miles, to get to the lake of Galilee! The storyteller was not concerned either to think about pigs which can swim.
Fun, huh. Nothing quite like the Romans being made fun of and oppressed persons wishing them driven into the sea where they belong.