Might be with autoresearched techs. Basically
Gobelin civ has a tech that affects a specific class eg corruptible and changes the visual actor. So when you capture another building with that class the tech apply and the building change.
I didn't try though so take it with a pinch of salt
Hey there you probably were playing Brittanic Road which has extra gaia units
You can make a mod to edit that map and remove the units or you can look at other maps
in the random category which is actually procedural you will find a lot of generic map that support any number of players from 1 to 8