Prior to the expansion, there will be pre-patch content that also brings a couple of changes for pet collectors.
1. Deathwatch Hatchling (reward for WoD Challenge Mode Guild Gold) will likely be removed from the guild vendor, similar to the Thundering Serpent Hatchling in MoP.
It won't be removed from the game completely, and will probably end up on the BMAH (same as the serpent hatchling). Still, I HIGHLY recommend collectors obtain this pet NOW, while it's still relatively "easy" to collect. There are a number of guilds that are accepting those looking to just collect the pet (see this thread), free of charge.
2. The WoD Digital Deluxe will be retired come Legion PRE-PATCH. This means the bonus rewards, including mount and pet (Dread Hatchling), will be unavailable once 7.0 hits. So if you want this pet, get it sooner rather than later. Post-Legion, the only source for the pet will be physical WoD CEs that you'll probably have to purchase from a third party website (Ebay).
Don't delay too long on these pets. The BMAH is a VERY unreliable source, and unless you're willing to shell out a good amount of RL cash for a physical CE, I would absolutely collect these two pets now while the collecting is straight forward.