How do you acquire Itherian creatures?

I noticed yesterday that I only have two Itherian creatures, and I think those are from way earlier in the game, when you could still accidentally encounter them. Now it seems every time I encounter one, it is in “this type of battle” where extraction isn’t allowed. Am I missing something?

They where extremely rare a long time, now they are more common again. I think it´s been a week that they gat renewed spawnrates.