A new patch will be deployed on the beta servers in the near future. Here are some of the more notable updates and additions that we've found!
The Feral Vermling was originally discovered as the reward for completing the achievement Mater Pet Hunter, however a new achievement tier was added, Zen Pet Hunter (Capture 150 battle pets). This newest achievement will award the vermling companion instead. [source]
It's a (Better) Trap!
Two new rewards were also datamined for pet battle achievements. A Strong Trap will be the reward for Going to Need More Traps (Capture 50 battle pets), while a Pristine Trap will be the reward for Pro Pet Group (Raise 15 pets to level 25). [source]
Currently these rewards aren't showing up in either Wowhead or MMO-Champion's databases, so this might indicate spell changes rather than actual items. We will have to wait to find out what exactly these upgraded traps are.
Is Three The Magic Number?
According to the updated client strings (PET_BATTLE_TRAP_ERR_5), it appears as though players will only be allowed to have 3 of the same companion at the same time in their pet journal in MoP. [source]
If this is true, players who have 3 deer pets for example, would need to release or cage at least one to be able to capture another wild deer.
Hopefully Blizzard's official blog post (coming "soon") about Pet Battles will have more details on how this will work.
MoP Beta: Achievement Updates
June 20, 2012