The WoD Digital Deluxe was removed without warning from the Blizzard store around BlizzCon 2015, and with it the bonus mount and pet.
However, Blizzard brought those items back for a special limited-time sale (ends before the Legion pre-patch - undisclosed date): http://us.battle.net/wow/en/blog/199906 ... 12-15-2015
To be eligible to purchase the bundle (includes pet and mount and other bonus items from the Digital Deluxe), you'll need to have an active WoW subscription and the Standard version of WoD.
The total price is the same as the original Digital Deluxe ($70). Standard WoD ($50, when it's not on sale) + bonus item bundle sale ($20).
So if you missed out on the Dread Hatchling
, this is probably your last chance to collect the pet.