As Guildox told us when they let us know about this, "it works similar to the battle.net site except for one major difference - we sort the pets based on rarity. It will show the most common pets/mounts that you don't have at the top of the list so you know which are the easier ones to chase."
This is such a great idea, and even as someone who isn't heavily into pet or mount collection, it's really interesting to see if there are any really obvious omissions from my current collection. For example, it seems that I haven't yet paid the 750 gold for the Reins of the Dark Phoenix, which almost 60% of players have, and that I should really look at getting my hands on some riding turtles, as over half the game's population has already done so, so it can't be that hard! If you want to see what you're missing, check out their new page.