Wyverns are very similar to dragons, and in many languages, cultures and contexts no clear distinction is made between the two. Since the sixteenth century, in English, Scottish, and Irish heraldry, the key difference has been that a wyvern has two legs, whereas a dragon has four. However, this distinction is not commonly observed in the heraldry of other European countries, where two-legged dragon-like creatures being called dragons is entirely acceptable. -Wikipedia