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

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 Fortress (lizardmen), there are rather poorly made basilisks (just hydras without heads), so if you give this creature a more or less normal shape + spikes and horns, it could become a decent unit for level 4/5.

Added 3 minutes later