Disclaimer: Be wary of phishing attempts on your account, and use discretion when opening any emails. Best way to check if you received an invite to the alpha is to log into Battle.net and check your account management page. On the right hand side (under the giant green "Add a Game Key" button) there's a link for Download Clients. Under World of Warcraft you should see a download link for "Beta Game Installer". Download, run that and your B.net launcher will be updated with an option to install and play the alpha.
Anyway, new stuff! It's not much but it's a nice start. Many of the previously datamined pets were added to the Pet Journal, with one new addition, the Albino Chimaeraling. Previews of the new pets aren't implemented in the journal yet; maybe we'll see them in the next update. Details of where each pet will be from were added, though. Keep in mind that they're still works in progress, and everything's subject to change.
Grovestrider Hatchling - Drop from Draenor
Lifelike Mechanical Frostboar - Engineering crafted
Iron Starlette - Engineering crafted
Mechanical Scorpid - Engineering crafted
Mechanical Toucan - Engineering crafted
Carrotus Maximus - World drop from Valley of the Four Winds (this is likely inaccurate and will probably be updated)
Lil' Leftovers - Cooking crafted
Weebomination - Unknown drop
Albino Chimaeraling - Looted from an egg from Shadowmoon Valley (Draenor)
Royal Peacock - Achievement reward (Draenor Pet Master)
All pets listed above are currently cageable but that will probably change.
Wowhead lists a few other new pets but I suspect that they are either NPCS or they won't be obtainable. Examples include an undead pet named Eleanor and what appears to be a clone of the Treasure Goblin, named Puzzle. Evil Widowling could be a new pet that wasn't included in the Pet Journal this time around but we'll see!
A notable new addition is Hogs which Wowhead currently lists as the in-game gift for WoW's 10th Anniversary. If it is it will probably be a small version of the infamous NPC Hogger. I'm thrilled to see this and I hope it's true! Any pet to help celebrate the upcoming anniversary is very much appreciated. :)
Other than the new pets, there aren't a lot of Pet Battle updates in this alpha build. There's a small UI change when you target a wild pet and then right click its portrait.
It's not a huge change and there aren't any new options in the drop down menu. We'll see if there's more to this change later down the road.
That's it for now - I'm sure I've missed something haha. I'm really excited to be on the alpha and I can't wait to discover and test more pet stuff soon!