Recently Pet Battles were enabled on the MoP beta and garnered much attention. Unfortunately, developers weren't quite ready for testing on this new feature, as Zarhym states in an official blue post. [source]
"While we've received a number of questions and some feedback regarding the pet battle system added in the last beta patch, we're in the process of disabling this feature. We know a lot of you are very anxious to test out pet battles, but it wasn't our intention to introduce them to the beta at this stage, as we don't yet feel they're in a solid enough state to allow for meaningful feedback.
One we're further along in development, we look forward to implementing pet battles and gathering your thoughts on the feature."
Pet Battles Will Return
Although battling will be temporarily turned off on the beta, Cory Stockton, a Lead Content Designer for WoW, tweeted the following messages in regards to Pet Battles:
"We are hoping to get it back on the servers soon, it's just way too early for meaningful feedback at this point." [source]
"I am working on a blog with lots of info on the system now. Hoping to have it up soon!" [source]
If you were unable to try out Pet Battles this time around, there will be plenty more opportunities in the near future for testing out this new feature! Also keep an eye out for an official Blizzard blog post about Pet Battles.
For those curious about what was discovered during this short preview of Pet Battles, head on over to the Pet Battles section of the WarcraftPets forums. But please keep in mind that testing for this feature was premature, so information gathered might not reflect the final version of how the system will work.
MoP Beta: Early Pet Battles Preview
April 26, 2012