There's a good deal to do with the creatures in your Menagerie.
Creatures come to you as fresh and new babies. They are pretty nondescript in appearence and won't reveal their adult colors until they reach level 3.
While looking at each creatures colour you will notice a number [R~?] This is their RARITY number...The HIGHER the number, the more RARE that type of creature is.
~Some newly discovered creatures have yet to be 'evaluated'~