Wyverns, drakes and leviathans are the three life stages of the same dragon species. Wyverns are juveniles who eventually grow too heavy to fly and become terrestrial drakes, which then continue growing until they are too heavy to support their weight on land and become fully aquatic leviathans. When the time comes for them to breed, they lay eggs close to shore that wash up on the beach and hatch into tiny wyverns, starting the cycle over.