Howvern

Howverns are the genetically flawed fertile wyverns born from a normal wyvern with a birth-infected virus and this infection passes on to other wyverns through sexual intercourse by mating with normal wyverns by mimicking the movements of an aroused female wyverns but the howvern gene will dominate over the wyvern gene and it will be passed through the male, infecting the wyvern gene pool and create a new generation of howverns (basically Howverns are born from Wyvern AIDS) 

Howvern's lifespan are half of their heather kin but they can still eat and hunt like they normally would except they are bound to hunting on land as their wings are underdeveloped and virtually useless