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

Okay, let's continue with the clarifications. The foot should be positioned at zero, but which axis should it be rotated along?
Regarding the converter, could you provide some guidance? I tried one, but it didn't work.

And could we create snake creatures based on the phoenix (but smaller) and make several of them, like ghosts (so that one unit is a swarm of snakes)? Accordingly, one of their abilities would be a chance to avoid physical damage.

I'm not sure about the axes. When I converted the Rakshasa to obj, it was facing down in Maya, but in the game, it stands facing forward. Ideally, it should be as it is in the game, with the foot pointing in the correct direction, but I'll clarify later.
There's a converter called ARCHANGEL.
If you reduce the size of the phoenix in the game, you won't be able to attach anything to it, but you could try creating a large, feathered snake like the one used by the Poles.

Added 1 minute ago
If you have time, I'll upload the converter in about 5-6 hours.

Added 5 hours 25 minutes ago
Okay, I've uploaded the program here: https://yadi.sk/d/tufm5ZaGpubTg
It turns out that you can't convert obj files to the Heroes format using this, but models from Heroes can be converted to obj