RTF is a global modification that replaces all towns in the game.

Because no one involved in the mod development can get the models from G6. The mod for the Nagas was created by another person (if I'm not mistaken, a Pole).

You are wrong, and very wrong at that. The Poles simply borrowed it without asking – we had to gently point this out to them. They generally like this style, but they are basically decent guys. I communicated with some of them for some time, and I still communicate with two of them – I gave them a few models from G6!

Just for your information – the Nagas from G6 are the merit of the respected comrade zAhAr.

Added 1 minute ago

How did they manage to remove the horse from the Knight and give it to the people? Is it even possible to remove and add something like that?

Added 5 minutes ago
Where is it actually written about this horse for the Paladins? How do they remove it, and how do they do it in Paint? Do they draw it themselves, or are there templates? And is it possible to preview the 3D image before installing it?

Added 34 minutes ago
Where do I need to go, where can I edit files with Paint?
You can remove something and add another something using an effect!

Added 7 minutes ago

If, for example, I want the skeletons to burn with black fire instead of green, do I need to write "dark fear" there, through splines and various brackets?

Not always. In general, the color of the effect is mostly embedded in the effect file itself, and then any OTHER color in the texture is overlaid on the embedded one – it is controlled here:

false
30072015-0000-0000-0000-300720150001

to

true
30072015-0000-0000-0000-300720150001

Again, this does not always work, and therefore you have to repaint the RGB color components of the NEW (for the game) file 30072015-0000-0000-0000-300720150001 in a hexadecimal editor – which is placed in Bin\effects\, but in the file itself, they are encoded in reverse, i.e., BGR