Before you can even go out and start killing a bunch of mobs in hopes for a special loot bag, you will need to do some prerequisite quests to gain a buff that will allow you to see the bag on the creatures.
The quest chain starts in Eastern Plaguelands, from the NPC Fiona. She stands just on the side of the road at the entrance of EPL from Western Plaguelands. Cross the bridge and you'll see her with her horses and carriage.
She will then send you on two quests to find her companions. Each one will then have you go on two more quests (one for each NPC) before having you return to Fiona. After completing all the tasks requested of you, she will give you a quest to "hop into her carriage." You do NOT have to accept or complete this quest in order to receive the buff necessary to farm for Mr. Grubbs.
Fiona's carriage should have a blue question mark above it and upon clicking on her carriage, it will offer three buffs. Choose "Fiona's Lucky Charm" and it should give you a buff with a blue rabbit's foot icon. This buff has no timer so I'm guessing it has an endless duration so long as you remain in the zone.
Once you have that buff, you are good to go kill yerself some grubs and devourers!
These gigantic yellow worm things are actually quest mobs and therefore don't really drop anything of significance if you're not on the quest. I had a low level warlock join my party to loot the quest items while my level 82 annihilated everything. Because the lowbie didn't have the buff, they weren't able to see or loot the bag. And since the loot bag (called Hidden Stash) is not a 100% guaranteed drop, having someone else loot the mobs helped me see which ones had a loot bag on it for me to collect. This meant wasting less time bending over to check each body for a bag. (yay for efficiency!)
Anyway, this is where the mechanics of farming for the Disgusting Oozeling come into play. The bag itself has a low drop rate, nevermind the chance of the pet actually being IN the bag that you've looted. Most bags contained a few copper to a few silver. In total I spent about 3 - 4 hours running around killing giant yellow wormy things. I can't supply hard numbers or even an estimate because after the second hour, I pretty much went into autopilot, one track mind, pet machine-mode. Talk about tunnel vision. lol. The number of mobs you have to kill may vary, but be prepared to kill A LOT of mobs.
Some last notes - the mobs that drop the bag are level 40 to 42. The hidden stash is bind on pick up. Mr. Grubbs is also bind on pick up (unlike the Disgusting Oozeling). And as with ALL beta items and companions, there is NO GUARANTEE that this pet or the way it is obtained will go live. there may be other ways to get this companion or the current way may change come expansion release.
In any case, I definitely hope Mr. Grubbs makes the cut and we get a chance to farm for him in Cataclysm! He's an absolutely awesome (and athletic) non-combat pet. I've always wanted a miniature grub. :D
And YES! True to this video, Mr. Grubbs will randomly do back flips up and over your head!! Sorry for the awkward screenshot. It's quite difficult to capture this little guy in action with just one image.