Homogenous means as-one-piece. Meaning the fog rolls in on the map, not as a fluid but as something that takes place on a per-biome level. I've previously examined above why fog-as-a-per-tile-fluid cannot possibly work.
You wouldn't notice the fog creatures, because they would look just like the fog. They would be the agent through which the fog destroys the structures. From the standpoint of the player, the fog rolls in, and the FOG starts destroying the buildings. The player knows nothing of these fog creatures, because they're just the way in which the GAME does exactly what you're suggesting.
This method is the easiest to implement, because it uses already existing in-game mechanics to achieve things, instead of requiring the air itself to be somehow changed in order to be an agent.
As per the idea of different fog critters, I love it. Some would attack those wandering through the fog, only to disappear and be replaced by another before you can blink. Others would wander, sneakilly, out of the fog, like ambushers.
The thing they share, tho, would be that they could all look just like the fog. Meaning that from the perspective of the player (nevermind how the game is working mechanically), the fog is killing you, and destroying your buildings, and reaching it's tendrils into your fortress and killing off your dwarves.