The pet is not really rare, you get it automatically with either a gift or defeating wizard city/unicorn way. I am not %100 sure but I think you get it for completing unicorn way. So it's not rare, because pretty much everyone is past Unicorn Way.
I'm not a pet or drop expert so I wouldn't know how to classify a pet as being rare. This pages gives information regarding the Blue Ghost pet, http://www.wizard101central.com/wiki/Pet:Blue_Ghost hope this helps!