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It's not necessary to make a mounted hero; you can make a foot hero. Take, for example, the Forest faction archer as a basis and work on it significantly. This will take much less effort in terms of attaching animations for shooting a bow and mounting a horse, but you'll definitely have to work hard on the body structure.
Stop talking nonsense; the male Stronghold hero model is much larger than the female Tower hero model, so the bow will hang outside of her body!
Another option is more difficult to implement... I started working on it, but abandoned it because the models and animations overlap in an unsightly way, meaning, for example, the bow can easily go inside the heroine's body or her horse, which is not good!
Added 6 minutes later
Why is a huge hero needed (the Overlord says hello)? Moreover, there is always an aura around the hero from the faction you belong to, so you can't "not see" him, and you would have to be blind not to notice the hero. Besides, there is Biara, who is not much different in size, but the developers didn't think that a "not large" hero is bad, so I don't see anything wrong with that. Increasing the size a little would be good, but no more than that. It's better for the hero to be on foot and not the size of half a mountain, but he will still be present in the game. Also, heroes on horses are not that big (without the horses themselves), so a small size for the hero would be quite logical. Besides, you yourself wrote that the hero's appearance is not very important to you, so what's the problem? Or is it better to struggle with the geometry and connect the horse, rider, and bow shooting animation from a third object? In my opinion, these are some unfounded stereotypes about heroes not being as large as a Castle. If we talk about the hero suggested by the girl above, I can share a few variations of textures for this hero so that you have a more accurate idea of what needs to be done. The textures are not based on the creatures/heroes of this game, but they will give a more than clear example.
When Biara is not your hero, she is really hard to see on the map; I once stumbled upon her a couple of times! I had to go back a few turns and use the [Alt] key to keep an eye on her!
С уважением, Psatkha (Псатха) 
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Неудачное планирование ведёт к неудаче ...
NCF - мод, добавляющий дополнительных существ в HeroesV 3.1

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Неудачное планирование ведёт к неудаче ...
NCF - мод, добавляющий дополнительных существ в HeroesV 3.1
