You might have seen people with this pet, the Terrible Turnip (mine is called “Ivan”, because “Ivan the Terrible”).
If you want it, don’t bother buying it on AH. It is very easy to get for free.
The only thing you have to do is to go to Halfhill village that you will find in the center (almost) of Valley of the Four Winds on Pandaria (you will have to be level 85 at least).
Now, go and talk to Farmer Yoon. You will find him near the ranch (which will become yours, kind of). And if you haven’t done any of the quests he offers, then time has come to do them.
Keep on doing the quests he gives you until you get your four first plots of soil.
Then, you just have to plant any seeds daily and harvest them when ripe (after the dailies are reset, they can be harvested). You can as well do the daily quest Farmer Yoon asks you each day and just plant and harvest the one he wants, then you’ll get extra gold (and XP if you are below 90).
When you do this daily, you will get an “Ominous Seed” when harvesting. Plant this seed and it will turn out to be the Terrible Turnip. It happens generally very quickly, you might get it after your first harvest.
Have fun.