I see a '!' badge in the Fiend menu/on my Fiends. What does this mean?

If the Fiendex or your Fiends have an exclamation mark (!) on them, this means that you currently have enough Meteormites, or Meteormites and other resources combined to grow a Fiend.

 

Once you have grown all of the Fiends you can, the (!) will disappear until you have gathered enough resources for another Fiend upgrade.