A Wyvern is a two-legged dragon with wings and, if you listen to people who play Dungeons and Dragons, a poison tail. The medieval wyvern had a barbed tail and this is sometimes depicted this barb in the same manner as a scorpion's. Another mythical creature that is similar to the wyvern is the cockatrice.
Many people have a wyvern as part of a device and it was seen fairly often in medieval heraldry, where it is often referred to as a dragon. Because of this, the SCA does not give count a point of difference between wyverns and dragons.