"The Roman legion was the largest military unit of the Roman army, consisting of 5,200 infantry and 300 equites (cavalry) in the pre-Marian Reform Republic (509 BC–27 BC) and 5,600 infantry and 200 auxilia (27 BC – AD 476). It was a highly disciplined, well-trained and heavily armed body of up to six thousand men who shared either a barrack room or tent."

At the time of much of this writing, the Romans were very much "top of mind".

It wasn't the name of the Demon. They were called legion because it's a large group of demons inhabiting one person, they didnt have one particular name.

What about when Jesus had to cast out all the demons into a herd of pigs from two people?

Mark 5:13 "So he gave them permission. And the unclean spirits came out and entered the pigs; and the herd, numbering about two thousand, rushed down the steep bank into the sea and drowned in the sea."

Matthew 8:28–34 "And when he came to the other side, to the country of the Gadarenes, two demon-possessed men met him, coming out of the tombs, so fierce that no one could pass that way. And behold, they cried out, “What have you to do with us, O Son of God? Have you come here to torment us before the time?” Now a herd of many pigs was feeding at some distance from them. And the demons begged him, saying, “If you cast us out, send us away into the herd of pigs.” And he said to them, “Go.” So they came out and went into the pigs, and behold, the whole herd rushed down the steep bank into the sea and drowned in the waters."

I found this when I was searching for Matthew 8. https://www.gotquestions.org/demon-Legion.html