NHF – new heroes for Heroes V 3.1.

Instead of adding, it only replaces, which isn't really desirable.
If there isn't some kind of script ID, then it's not necessarily true that the slot isn't occupied;)

On some standard maps, there are definitely Black Knights in the neutral creatures on the paths (like Key or something else, I don't remember exactly). Replacing them will change the unit.
Yetis aren't used in standard maps.

Hmm... I've seen Death Knights in neutral creatures many times, but I've never seen a Black Knight while playing – only some files in data.pak.
Maybe we're misunderstanding each other – the Death Knight (texture color is black and red, #90) is in neutral creatures and is often used, but the Black Knight (texture color is black, #89) is not used at all (I haven't seen it in either custom maps or standard maps). If it were used, that would be one thing, but when it exists and no one has seen it, you can say that it doesn't exist at all – please provide an example of a map where you saw it; I'll specifically remove all mods and maps from the game and try to see it in the original game