Mark, the writer who came up with this story, wasn’t writing history. He was writing something more like Shakespeare, and this passage is an allegory.
Mark’s first language was not Greek. Scholars can tell this because the Greek he uses is “difficult”. If you read Mark in Greek, almost every sentence literally starts with “Immediately” or “And”. Imagine reading a story that’s written like this:
And then I went to the store and bought some eggs, bacon, milk, salt, pepper, and potatoes for breakfast.
Immediately, I went to the counter to pay for my breakfast.
And the store clerk asks me if I have bills less than 20s since he’s running out of smaller bills
And I pull out two 5s from my wallet so that I can help the store clerk out
And I paid for my breakfast and headed towards the exist
And I ran into an old buddy of mine while at the door
And he said ‘hi’ to me and asked me what I was doing up so early on a Saturday
And I told him that I had a long day of studying ahead of me so I wanted to get a head start
And he said that this was uncharacteristic of me since I was known for partying and staying up late
And I told him that I wanted to try being more responsible since I’m currently on academic probation
This is what it’s like reading Mark in Greek. So tedious.
Anyway, Mark might have a Roman background of some sorts, since he uses a lot of Latinized Greek words. Like “centurion”: It literally means “leader of 100” in Latin (think of the word “century”). Matthew and Luke use hekatontarchos, which is the Greek word for “leader of 100” (e.g., a hecaton, a 100 sided shape).
Mark also provides Latin translations for some reason:
Mark 12:42: λεπτὰ δύο, ὅ ἐστιν κοδράντης — lepta duo, [which] is a kordrantes (Latin “quadrans” the smallest Roman coin)
Mark 15:16: τῆς αὐλῆς, ὅ ἐστιν πραιτώριον — the aules, [which] is the praitorion (Latin “praetorium”)
With that said, there’s a reason Mark chose “legion”, which is also a purely Latin word. Moreover, there’s a reason that Mark chose to associate pigs with this “legion”.
X Fretensis symbols were the bull — the holy animal of the goddess Venus (mythical ancestor of the gens Julia) — a ship (probably a reference to the Battles of Naulochus and/or Actium), the god Neptune, and a boar.
Legio X Fretensis was responsible for occupying Jerusalem after the Jewish War (ended 70 CE), staying into the fourth century. After 70 it was stationed in Gerasa for a while (Winter 1974, p180-181). Allusions to the Jewish War with Rome in Mark are the reasons why many scholars date the composition of this gospel to around or after the Jewish War, which, again, ended in 70 CE.
Geographically, having the pigs run from Gerasa into “the sea” makes no sense. Gerasa is about 30 miles from any large bodies of water. How long do you think it would take you to run 30 miles? Were Jesus and the poor recently possessed man standing there for a few hours waiting for the pigs to get to the sea? Matthew recognized this mistake and changed the location from Gerasa to Gadara but Gadara was still six miles from the lake.
So to recap, we have the 10th Legion Fretensis who were stationed in Gerasa after 70 CE and one of their standards was a boar. We have this story, written sometime around or after 70 CE where a guy goes to Gerasa and encounters a demon called “Legion”. He casts the demon into a herd of pigs and the pigs leave the region.
This is an allegory about kicking the 10th Legion out of Gerasa if I had to put betting money on it.