That would be a bug related to the Mage glyph, Crittermorph.
"When cast on critters, your polymorph spells now last 1 day and can be cast on multiple targets."
Sad to say, it'll probably go away
My own "lucky catches" are several Hyacinth Macaws on the AH for under 100g, Mojo, Phoenix Hatchling, and a few of the Argent Tournament pets for under 100g, Magical Crawdad for 23g, the kites for 5g, and a bunch of others.
Actually battling, though, a recent entry:
I was running my Tiller dailies and realized that I was tired of the poor frog I'd been using as backup so was attacking turtles along the shoreline where the Enormous Grouper swims. I end up getting into a battle with 2 rares, accidentally killed the first, then with 2 dead pets and one of those big mogu guys pathing painfully close to me ended up catching the second.