Rare Egg can be bought for 10 Crystals from the Get Pets page. This kind of egg will hatch B or A grade Pets, or possibly 10,000 coins if you already have at least one B or A grade pet at Level 8. You will also receive 33 gift points for each purchase. You may receive an upgrade of a Pet you already own, if it's not at level 8 yet.
In the first two seasons, rare egg can be rewarded by inviting friends in the Invite Event as a replacement from the Extra Rare Egg. However, in the third season, it is replaced back by the Extra Rare Egg.
List of Pets obtainable or upgradeable from Rare Egg Edit