I like Cassius Dio's account just about as much as I liked (i.e. disliked) having to translate that same dragon story in Livy. In high school Latin class we had to take turns translating Livy every other day and Julius Caesar's Gallic Wars on the other days. I was a crazy teenager who thought Latin would be good for me. (OK, I suppose it was but I didn't enjoy it much.)
I assume you understand that both Livy and Cassius Dio reported those alleged dragons stories centuries after the fact, right? Military heroes and even average soldiers loved to tell exciting stories of their exploits in far away lands. Much like fishing stories, everything get bigger the more it was retold. And whenever you see words like "monster" and "dragon" in an ancient text, you should keep in mind that these words basically mean "big and scary animal". (The first European to see a war elephant in full battle armor called it a monster.)
The same can be said in modern times. When Indonesia was a Dutch colony, some soldiers thought that local tales about "dragons" were just mythical stories---until they saw one with their own eyes carrying off a young woman. They were on the island of Komodo and saw the scariest giant lizard that they could ever have imagined. The name stuck. Even many American zoos today keep a few Komodo dragons around (although most don't have the huge adults which are quite big but apparently are not nearly as large as they used to get back in the 1800s before major habitat destruction on the islands.) Yet an untrained person could be forgiven for calling them "dinosaurs" or "monsters". Indeed, Komodo dragon suits them just fine. What was the animal in the stories which got passed down to Livy and Cassius Dio? Nobody knows. Did it actually exist? Nobody knows.
Your question reminds me of a story told by one of my linguistics professors as a cautionary tale. He said that some ancient historian (I can't remember what culture) told an exciting story about an army having to fight some kind of "100 legged monster". It was described as two hundred feet long and twenty feet wide. It was covered in thick-skin and had never ever been defeated. Yet not until centuries later did scholars figure out that it wasn't a monstrous animal at all. No, it turned out to be the name of a "military formation"!_ Military tacticians called it the "1000 legged monster" because it was composed of 50 foot soldiers in a tight formation of ten rows or similar, with their cowhide shields held outwards. (This should remind you of a somewhat similar Roman testeudo, turtle formation.) That story serves to remind us that not only can "dragons" and "monsters" be virtually any large fearsome animal that is foreign to one's culture, it can even be something which has been confused by language barriers!