Mr. Grubbs
Drops from Hidden Stash. The grubs of the Plaguelands are a side effect of necromantic corruption and the massive number of corpses there. Mr. Grubbs is from a particularly... Beast type. Mr. Grubbs is a companion, you get him from by having the buff from Fiona's Caravan -while slaying mobs and looting them, they will then drop the Hidden Stash bag.
Mr. Grubbs is a companion pet dropped from the 
Hidden Stash, a bag that only drops from slain mobs in Eastern Plaguelands while you have the buff from Fiona's Caravan active.
Guide awaiting review
Obtain Fiona's Caravan Buff
Travel to Eastern Plaguelands and find Fiona's Caravan to pick up their buff. This buff is required — without it, mobs will not drop the Hidden Stash bag.
Farm Zul'Mashar Undead Trolls
Head to Zul'Mashar in the north/northeast of Eastern Plaguelands and slay the undead trolls there. Zombie trolls also spawn from graves, providing a dense cluster of mobs to loot for the Hidden Stash.
Loot Hidden Stash for Pet
Loot every mob you kill — the Hidden Stash bag will occasionally drop, and 
Mr. Grubbs can be found inside it. It is possible to obtain multiple copies, and any extras can be caged and listed on the Auction House.
Tips
Mr. Grubbs belongs to the maggot family and can deal significant damage in pet battles by combining Acidic Goo with Consume, which also results in substantial healing.
On the cooldown round for Consume, using Leap makes Mr. Grubbs faster than the opponent, giving the next Consume a better chance to land before being attacked.
