Calie,
this is the list thread on CS,but I wouldn't like for that system to migrate here,to be honest D:
While it's good to protect people from unfair trades with a rarity system,as well as guides telling you in which date a certain creature was released,which type it is ( common,paladin,etc. ),I'm against a list that would be treated as gospel like that one is in CS.I may be in the minority,seeing as it's so popular over there,but I think it negates the freedom and fun in trading for those pets.
Also,getting really old and rare pets isn't as seemingly impossible here as it is in CS,thanks to the events and the exploration,for which I'm incredibly glad and grateful.
I don't know..maybe a bit of clarity,explaining which among the rare/very rare/ultra rare are the most sought after creatures or the most difficult to find in trades would be helpful,but a closed list like that,I wouldn't like.Just my opinion.