Astelnair is a mod that adds new creatures and much more.

Only textures, but you can experiment with them in any way you like.

You can change creatures, costs, the transformation system, and even the buildings themselves.
You can't add new ones.

Added 1 minute ago
You can create as many new creatures as you want using old models with new textures.

Oh, and here's another question. I saw a simple method in the example of the second game. There, in the Undead faction's dragon branch, after the dragons, there was a continuation, but the building was the one needed for another tier (or rather, its texture).
Essentially, nothing changed; it was just two textures in one place. Is that possible?