Originally posted by Vazkulator:

id avoid killing abomination until youve got a relatively secure base since it appears in the wiki that killinga ♥♥♥ will have new spawns that include air units if im not mistaken

i don't recall abomination being one of those triggers, but there are a few critters that trigger their own raids (like trolls or bats) if you have progressed far enough and have killed one of them. Troll raid is the first big pain in the butt for many, because of the sheer reach of the troll normally (they can hit through earth walls, even if they cant pass them). Bats is the first flier group you can unlock in raids I believe though, and that's "kill a bat, and beat bonemass" for requirements.

Although it is worth mentioning that some progression advancements do in fact spawn critters in weird places. Like after you beat the Elder, before you even visited a swamp the first time, you can encounter Draugr in the meadows during foggy nights (they despawn at dawn, a lot of these are night spawns that disappear at dawn, like you can also get fulings spawning in meadows at night after you've beaten Yagluth as well).