You’ll first encounter Guardian Arkveld in Chapter 3, during “The Faithful Keepers” main mission. The definitive fight won’t take place until the “Broken Chains” mission. As such, all you need to do to unlock the monster is progress through the main questline.

Monster Hunter Wilds' Flying Wyverns are the most common Wyvern type, at least until Capcom adds new monsters and Wilds' expansion. These are typically Apex Predators, the monsters who put up the biggest fights and rarely back down from other monsters if they pick a fight. They're also excellent sources of materials for crafting powerful equipment.