I was very lucky to hatch one of my favourite creatu as achromatic this month - the Meragon. But I know that only certain creatu can be hatched as achromatic in a month. Is there a list somewhere that lets us know, or a specific update? Thank you!

There was an update some time ago allowing all pets to be hatched achromatic at all times. The only exception is non-mythical CS pets, which still have a schedule and whichever is currently in the CS is the one that is hatchable as achromatic. I believe this explanation can be seen if you click on "Hatch Color Chances" in the hatchery as well.