In the context of Pact of the Dragon, 'wyvern' is the term I use for wild species of Dracotherium, while 'dragon' refers to the population of domesticated mountain wyverns that have since gone feral after the collapse of the dynasty of witches that made them oversized familiars.
Keenan Taylor
Though feral dragons are primarily derived from the now-extinct mountain wyvern, the dragonriders are said to have incorporated both green and common wyverns into their domesticated stock for a range of traits.
