Like I said... "in my book." There may be many mythological sources that disagree with me on this, and my distiction may be based more on fantasy literature by contemporary authors than documents based on historical superstitions...but at the same time these models seem to be based on the same literature and corresponding artwork for the most part But here's how I figure it: 4 legs, 2 wings = dragon 2 legs, 2 wings = wyvern 4 legs, 0 wings = wurm However this varries depending on which fantasy series one is most familiar with, so this is by no means a hard rule.