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

They stand, the Thunderbirds, in a cloud (the texture of their attack). The outlines are vaguely visible, with a few heads, hooves, and hands.

No, I think that doesn't look very good. It would be possible to create some kind of electric unit, for example, take a Djinn, name him "Lord of Storms" (or something else), add a few clouds and electricity, and have him attack like a Succubus, in a chain, but with electricity (if that's possible; if not, you could just put Chain Lightning in the spellbook), immune to air and lightning, flying, not many hit points, medium damage, incorporeal, possibly not vulnerable to earth.