So what will 6.0 have to offer us pet collectors? At least two new pets!
The two confirmed pets are the "Iron horde" pets, the Iron Starlette and Bronze Whelpling. You'll be able to collect these pets by participating in the pre-expansion content. Complete the quest chain for the starlette and kill some (or a lot) of Ironmarch orcs in Blasted Lands for the whelpling. Both pets are cageable.
Other pets that MIGHT be available in patch 6.0, but haven't been confirmed yet. The availability of these pets will all depend on whether or not the patch releases the new content for each in-game event.
- Cursed Burman and Widget the Departed - New Hallow's End pets
- Syd the Squid - New Darkmoon Faire pet
Grommloc, the BlizzCon 2014 pet, should also be available prior to the expansion's release since the convention is before the release date, but we'll see. I've got my Virtual Ticket, so I'm ready for at least that! XD
EDIT: Stormwing may also be available to collect in 6.0. You get this pet for completing So. Many. Pets. (collect 600 unique pets). On the PTR this achievement is present, but we'll see if it's released with the patch on live servers.