Future expansions.

I'm completely confused with the names.

There's nothing to be confused about. There used to be the Medusa, now it's been renamed to Coral/Pearl Priestess, and they've given it the ability to shoot and resurrect allies. The River Spirit and Mizu-Kami don't have ranged attacks, but they can establish a spiritual connection with allies, sharing various kinds of influences with them. The Snow Maiden/Yuki-Onna can shoot, and after an upgrade, she can also summon ice blocks from the ground.
And there's nothing complicated about it.

And besides, how does this relate to the topic?

Like this. Just because these creatures are similar in style and concept doesn't mean they'll be turned into a faction. They could remain as neutrals, which is what they are in the game right now. That's why you're being asked where you saw a neutral faction city in the third game. In terms of beliefs, yes, there is one. In terms of composition, no. When forming armies in the third part, the main criterion was location, so neutrals could also have been combined into one city, but this wasn't done. In the sixth game, they could simply add neutral units and not form a separate faction. But we'll see; if they do create such a faction, it will be quite appealing. I just don't really like the style.