Hi! I'm fairly new to the game myself but I've figured out some things that might help you! If you click on the "Guides" tab and then on "previous series" you can check all the creatures that have been released since 2010. As Muna said, each month you can get 4 dif creatures of each species (Destrinos, Flamiris, Lunaris, minoushas and Stoufix) and special quests may give you special creatures.
In the guide, next to the creature you'll see a plus sign. If you click it you'll add that creature to your wishlist which lets other players know what creatures you'd like to have. You can trade your duplicate creatures for those in your wishlist.
As for rarity you can check on the symbol by each creature or in the trades tab. When you select a creature to trade you'll see a little colored bar under it. Green means rank 1 which is the most common creatures, yellow means rank 2, orange rank 3 and so on. Hope I could be of some help!