First is a datamined scenario currently titled The Celestial Tournament. As of now, the achievement for this tournament is listed under the Pet Battles subcategory, so it's very likely the pet battle tournament mentioned in the patch 5.4 patch notes. A new battle pet named Essence of Xuen provides some hint as to the rewards possible.
In addition, Crafty and Rascal-Bot have appeared among datamined pets. Crafty currently appears little more than a placeholder -- its icon is a human male and the pet possesses a tooltip that reads: "Right Click to summon and dismiss Crafty. This pet is awesome." If Crafty is a pet that will be hitting our collections in the future, it likely will not represent its current state.
Rascal-Bot is more interesting: it seems to be a revival of Pierre, the engineering pet that never made it to live realms. Rascal-Bot's tooltip reads: "Right Click to summon and dismiss Pierre. If you give him a moment to heat up, he makes an excellent cooking fire."
MMO-Champion has datamined the models for some of these pets, as well as models for pets that have not yet appeared in the datafiles such as a miniature version of Chi-Ji and the Jade Serpent. It seems likely that we may end this expansion with a kennel full of baby Celestials.