Posts from RTF is a global modification that replaces all towns in the game. Auto-translated

It's up to you :)
Feel free to experiment :D

Added 5 minutes later
By the way, here's another idea:
I think it would be good to create Orcish warriors based on Minotaurs.
But there are already Murmillo in Haven. So, I think:
1) Remove Murmillo and create Orcish fighters based on Minotaurs.
2) Move Kensei to level 5.
3) Put Hydras on level 6, but only with a tail and not legs :)

Based on which creature should the Hydras be implemented?

Actually, hydras :)

Added after 37 seconds
You could take them from Might and Magic VII, recolor them, and voilà.

It's up to you :)
Feel free to experiment :D

Added 5 minutes later
By the way, here's another idea:
I think it would be good to create Orcish warriors based on Minotaurs.
But there are already Murmillo in Haven. So, I think:
1) Remove Murmillo and create Orcish warriors based on Minotaurs.
2) Put Kensei on level 5.
3) Put Hydras on level 6, but only with a tail and not legs :)

Wouldn't it be possible to get trolls this way from the 6th game?

Did the hydras' tails atrophy after the cataclysm?

Hydras are generally creatures of dark forces... how will they look in the Sanctuary? Maybe it would be better to give them to the Lizardmen?

However, you are still limited by the laws of the Ashan bestiary.

Yes, I know that they appeared under the influence of chaos, evolving from ordinary snakes.

Added 1 minute ago
However, perhaps the Nagas performed some kind of purification ritual, and all the chaos was expelled from them? And now, their strength is no longer supported by Chaos, but by the magic of Shalassa.

Interesting. That could work, but hydras were still a great fit for the Lizardfolk.

Perhaps you have suggestions on who could replace the Murmillo? If the replacement is worthy and quite feasible, then the Hydras will be sent to the Lizardmen.

What about hydromancers? Mages who found a way to survive after the cataclysm by appealing to Shalassa and her servants?

I think it should be either nagas or water spirits, but definitely not humans, elves, etc.
1) We already have elves in the army.
2) Nagas already have sorcerers.

I also have an idea about a giant sea turtle... but it's not easy to implement either.

That's a good idea.
In the old mod for the Citadel (lizardmen), there are rather poorly made basilisks (just hydras without heads), so if you give this creature a more or less normal shape + spikes, horns, it could become a decent unit for level 4/5.

Added 3 minutes later