If there’s one criticism I have about the AC series as a whole, it’s that they don’t play up enough the fact that you’re playing memories, or impressions of the way things happened, and not some objective reality of historical events. In my opinion really only AC3 made a gesture in that direction with opposing impressions of the two main factions.
But maybe that’s what they’re doing here - the protagonist would be steeped in the lore and culture and religion of his time. He might well believe that things like giant snakes exist, and that might color his impressions and memories of events. I like to think so, anyway, but probably it’s just a big ass snake.