Once you purchase a pet in the cash shop, it's yours after 90 days. After 90 days, they'll turn into a doll as seen in your inventory in the Cash tab. To revive them you have to buy water of life in the CS. Go to Mars in ellinia and she will revive your pet.
Unless you buy a permanent pet from that event, that pet is forever yours (meso magnet, all the pet stuff you have) will never disappear