"240H" – unlocking new heroes and improving existing ones.

Is it possible to have a Knight ride a Nightmare?

Yes, as an effect! Meaning either the "mount" or the "rider" will be an effect!

Added 15 minutes later

I see, they even took some icons from NHF.

Yes, not just icons, but also the "inner workings" of the heroes: Tarsis, etc.

In general, it's something like "Nostalgia for heroes from previous parts"!

It's a matter of personal preference!