Well, personally, I think that most people fail to realize there are more than just four-legged dragons (and yes, they actually do exists, today). In fact, both exist, still, all over the world.
But two of the main differences between them is that while most (known) dragons have four legs to accompany their wings (which, mind you, not all dragons have either), a Wyvern (and I still cannot get myself to say that word like everyone else seems to) is said to have only two legs (as the front legs are "fused" with their wings), and the creature is also much smaller than its older counterpart.
Granted, they are much smaller, and in some ways more vulnerable/"less powerful" than dragons, they also have their own talents that make them a force to almost rival a dragon.
Mind you, I know that a lot of people are going to see my comment and likely call me "crazy", but hey... what's new?...
Frankly, I don't care whether people believe it or not, but there has also been evidence found to their existence, to-day. Which also happens to include having found out how some (mainly European) dragons were/are able to even breathe fire. They just haven't been able to find many of the others, that I've heard of.