It's become a popular speculative evolution trope for the likeliest candidate of the mythological "wyvern" to be a Cenozoic family--if not superfamily--of scansoriopterygid dinosaur.
Now the first thing you'd be wondering is, "scanso-WHAT?" and I won't blame you. The name is not as memorable or catchy as "Pachycephalosauridae". So the "wyvern family", as I've decided to call it, was a group of small, arboreal theropod dinosaurs known in only the past decade or so for going around with batlike wings, demonstrated in this gorgeous portrait of Yi by Emily Willoughby:
But there is a problem with this family, one that has been criminally overlooked and frustratingly unanswered--the wyvern family, in our timeline, was hardly successful. Only four species have been unearthed from Chinese rocks, and their legacy was unnoticeable, existing from 165 to 156 million years ago. This is a big deal because the family died out at a relatively quiet point in the Late Jurassic period, rather than a sudden, dramatic catastrophe like the one that paved the way for the dinosaur empire 45 million years earlier or the one that'd end it 90 million years later.
While other people have been asking on how to make the Cenozoic wyverns biologically and physiologically plausible, mine is on how to make them chronologically successful. As in, what point of departure would I need to ensure that the wyvern family made it to the Cretaceous, survived the fall of the dinosaur empire, endured the unrelenting climate changes of the Paleogene and Neogene and thrived long enough for the knights in shining armor to fight them off?