As I play the game, I notice that eventually tougher enemies show up. From what I've read, it sounds like this is based on elapsed time rather than "research checkpoints", but I haven't seen any specifics. What kind of time frames are we talking about for various enemy types to appear? Are there any fixed enemy events that happen after a certain number of days? Does difficulty level affect this?
I want to have an idea of when I can expect so that I can better plan my strategy.
Best Answer
It seems that some enemies are tied to some missions, while others will appear anyway, but it seems that if you are quick enough they will appear earlier after certain missions.
Credit to yx and Sterno in the comments below !
Enemies which are tied to missions
Outsiders - First appear in a downed UFO. One appears in most (but not all) UFOs after that.
Sectoid commanders - first appears during the assault on the alien base. After this point, Sectoid commanders appear on the bridge of alien UFOs, replacing the Outsider. After the overseer ship mission is complete, they are replaced by Ethereals. Sectoid Commanders may also appear on harder alien abduction missions, usually in small groups. Source
Ethereals - First appear in the assault purple ufo mission, and then randomly in missions afterwards
Enemies which will appear anyway
Spartacus mentioned that Chrysalids may be tied to the first terror mission.