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I've said it before, I really liked the dragon in the sixth installment; it's a case where they experimented and did it right. In all the previous games, the dragon was more or less just a big lizard, but in the new version, it has a certain cosmic power (it can just grab a galaxy and smack you with it!).
As for the fifth game, yes, it's not the most successful design. In the game, it looks very dangerous, and it's not much different from a "pit bull," as dnaop-wr put it. At the same time, its proportions are much better compared to earlier dragon versions: in its case, you really subconsciously believe that it can fly; before, when the dragon was flying, it seemed like it was just slowly falling...
In my humble opinion, the fourth game's version is the least successful of all. It looks more like a frog, at best a toad, but it's definitely not a dragon.
I won't say anything about all the swords and magic, as well as Heroes 1; I think these dragons have nothing to do with the black dragon, and they were probably added here by mistake.
In Heroes 2 and 3, of course, it's a classic: a beast that sits in caves on treasures and eats people little by little. But in the third game, among them, the black dragon is the best, again, in my opinion, as someone who has played the third game a lot...