yeah, you must not have played for very long.
They also are prone to appear at the least oppurtune time, like when you have a city with nobody on it to be protected *BAMPH* a deamon attacks. That isn't entirely true, but that is how it feels because it happens (also because demons move by 'flying' so they shoot over water and similar places where you don't expect, so you THINK its safe until something comes from the sea. Also, deamons are tough and usually can't be stopped by the 1 spearman I sent to protect the town from other random monsters.
I figure this game would have settings for it, like how tha barbiarans and neutral animals had settings Civilization 4.