I’ve explored this before, but now I can actually confirm it. After weeks of going to the pyramid level daily, I have at least 5 of each pyramid except Pyrromid, whom I have only encountered once, on my 2nd go at the challenge, a long time ago. Since then I haven’t encountered a single one. It seems the devs have rigged it so that in each ingredient challenge, one monster has a blatantly lower drop rate. In the bottle missions, for me it was Halobottle. For some others in the pyramid challenge, it’s a different monster. For example I’ve seen some people complaining about not seeing Noxmid, who I have 5 of. (It doesn’t help that there’s only one encounter on the pyramid level either, and it costs 30 tickets!) This confirms that, varying from player to player, one ingredient monster in each challenge has an extremely low drop rate compared to the other obtainable monsters. The devs simply want you to play more and hopefully refill tickets using gems.
I think a good fix for this would be to change the monster that has a lower drop rate after a certain time, maybe monthly. That way I at least have a chance at evolving my monster, instead of it being impossible.