But yeah, I agree. There's a Pokémon called Sanaconda that was introduced in a generation where the main gimmick is that mons can become LITERALLY gigantic... I was expecting that mon to be the focus on this doujin (literally the only reason I clicked (and because of May) on it since I'm not into vore) only to be left out a bit disappointed.
Mist Night rarely uses predators that aren't snakes or monstrous worms. I think I remember one or two comics where Ekans or Arbok were used, but it's usually not a pokemon. I don't mind it too much, as I love snake preds, but it would indeed make more lore sense to use a snake pokemon. At least this time it was a fakemon.