Thank you to everyone that took some time to assist! Information on the two MoP rarespawns that drop a pet each can be found here
The Pet Journal was updated with more information on two datamined pets: Aqua Strider and Grinder.
According to the journal, both are BoP drops from NPCs found in the Dread Wastes (lvl 89 - 90 zone). I haven't had any luck finding the NPCs in Wowhead or MMO-Champ's database. I don't have a level 90 toon on the beta yet, so I can't fly over and do a search for the two NPCs.
If anyone has a level 90 toon (or even a lvl 89) and the time, please be on the look out for Nalash Verdantis
and Karr the Darkener
in the Dread Wastes. If you have any information on these two NPCs/creatures (Rarespawns? Quest mobs? Other? Location? etc.) please share!