So apparently Wyverns added by the mod become invincible, as well as bred Wyverns. I just raised a Fire Wyvern that is stuck at the same HP, Food, Stam, ect... as when it was a baby but it takes no damage and the stamina doesn't run out. Can literally fly and use breath attack forever with no repercussions. Downside to invincible Wyvern is that ALL Wyverns are invincible. Got into an air battle with a lightning Wyvern and after about 5 minutes of fighting I stopped to check its HP and it had lost exactly 0.