A Wyvern (or wivern) is a winged reptilian legendary creature often found in medieval heraldry where the usual blazon is statant (standing). The wyvern is similar to a European dragon, but it differs in that it has only two legs, cannot breathe fire, and has a barbed or snake-like tail…

Occasionally, it is said to be smaller than a dragon or to be able to breathe fire. In heraldry, it represents pestilence, conquest, or other such ideas.

text taken from wikipedia