First megabeast I had was a hydra. It took out the poor woodcutter I drafted to keep it occupied while the proper army ran over, but it fell quickly once the guys with training got there. Pretty underwhelming. It did come to my fort missing an eye, but it never seemed too hampered by it(then again, when you have 14 eyes, losing one doesn't mean much).
Second megabeast was a dragon. Despite the hydra letting me know that yes, vanilla megabeasts are exactly as underwhelming as many complain they are, I was still worried since dragons have a habit of breathing fire at creatures, which run around and set fire to other things.....
Somehow, absolutely nothing caught on fire. Not even the war dog it breathed fire at when it first set foot on the hill my fort's dug into(it started on the end opposite the fort entrance, thankfully), or the champion it breathed fire at, or any of the trees in the area...actually, the dog/champion didn't get hurt by the fire at ALL(and I do have temperature on, so it's not like that would make a difference). The only sign that the dragon had done anything was two swathes of grass turned to ash. Rather bizarre, but I wasn't complaining. Once the dragon was done not setting things on fire, it tore the war dog to shreds. The champion arrived about a minute after the dog was killed, kept walking towards the dragon that was breathing fire at him, and singlehandedly took it down just as the rest of the military got there.
Total megabeast death toll:A woodcutter, a dog, and some grass. Any new megabeasts who decide to come to my fort before the king arrives(at which point I will put several "end fort hilariously" schemes into action and get started on my second fort) probably won't fare much better, since lots of dwarves who don't have much to do anymore means a very large military guarding the fort.