only a few Crystals can be used on Eternal Items, Pets, and Maps.
here is a list of Crystals that can be used on Pets that aren’t Eternal.
if you are willing to wait 3 hours every time you don’t get the Pet you want, then you can go with the method @Deathbro shared. you could get lucky right away or end up spending a week or month waiting 3 hours to check your game for the right Pet.
you could also make an Eternal Pet Farming Build, like @Mr_Scooty , and try that method to get the Eternal Pet of your dreams. even if you don’t get the Dream Pet right away, you are still getting lots of Loot.