Mr. Grubbs

This pet is definitely worth the farm.
If you want one of your own, here is what you'll want to do.

First, fly to Eastern Plaguelands. You'll want to hit the point closest to the western border.
There will be a female Worgen named Fiona nearby.
She will have two quests for you. Gidwin Goldbraids and Tarenar Sunstrike.
You will have to find those two NPCs (in-game quest helper will pinpoint locations) and complete their quests.

Once you have finished that, a new quest will open up with Fiona. You DO NOT have to do this quest.
Instead, head over to her caravan, which should have a blue question mark hovering over it.
Of the three quests it offers, choose Fiona's Lucky Charm.
This quest will give you a buff that stays on you until you leave Eastern Plaguelands.

You are now set to farm this beauty!

My suggestion is to veer off the road and kill EVERYTHING you can on the way to Corin's Crossing.
Once there, firmly plant yourself in this small town. Everything, and I mean absolutely everything in this town can drop a Hidden Stash.
The respawn timer is extremely fast. I could never wipe the entire town out. There will ALWAYS be mobs to kill if you are by yourself.

I didn't count kills or use an addon, but it seemed to me that the Hidden Stash had a drop chance of about 10% and it took 26 of them for me to find the pet.

Happy hunting and GOOD LUCK. :)

I found Mr. Grubbs contained inside a Hidden Stash that dropped off of a Carrion Grubb in Eastern Plaguelands. To see the Hidden Stash drops you need to have Fiona's Lucky Charm active; available after you complete the quest of the same name Fiona's Lucky Charm.

Sorry I can't confirm that it only drops in the hidden stash; but this is how I got it. The buff does say it allows you to loot extra gold AND items, but that doesn't mean it's an item not otherwise obtainable.

Edit: A link to a video of Mr. Grubbs

Edit: I just got Mr. Grubbs again on a different toon. This time from a Hidden Stash that dropped from a Wretched Ranger, so I can confirm that it can come from a Hidden Stash that drops from any EPL mob.

This has one of the best icons ever now.