Which Might and Magic universe are you referring to?

Did the Dark Knights accidentally switch to the class of Heroes who ride horses?

They've been around since the third part. The Necropolis Warrior is a TR.

And where do you get so many dragons, and make sure they're in good condition?

They managed it in the Fifth installment. Or are you saying that in the 400 years between the 6th and 5th games, so many dragons were killed that it was enough to replenish the Necropolis army? Understand, you can't justify it with the plot; the developers simply didn't include them in the game, and later fans can come up with some kind of story to fit it. But the truth lies not in the plot, but in the gameplay.