This is the first post-Shattering pet I have acquired, and because getting this little guy is a bit more complicated than just farming or visiting a vendor, I thought I would post a really quick but detailed guide on how to get one of your very own!
To do this, you will need to be at least level 40. Head on over to the Eastern Plaguelands and visit Fiona, who is located right on the road leading into EPL from Western Plaguelands. She will give you two quests.
First do the quest “Gidwin Goldbraids”. You will find him by heading north along the river, just outside of Terrorweb Tunnel, in a web. He sends you into the tunnel to kill spiders for 8 “Crypt Bile” and then he asks you to bring him 10 “Banshee Bells” from the river bank. These can be hard to see as they are very small and look very “Plaguelands”, but they’re there!
Next, head South to meet up with Tarenar Sunstrike around (15,70). He’ll ask you to kill 5 “Death’s Grip Miscreations” which are just south of where he was standing. When you turn that in, he’ll ask you to collect 10 “Samples of Plaguehound Blood” from the Plaguehounds on the north side of the road. Then return to Fiona and turn in the quests.
You should see a blue question mark above her caravan now, so get yourself the “Fiona’s Lucky Charm” buff. This will give you a chance to loot a “Hidden Stash” from any mob you kill in the Eastern Plaguelands, and your very own Mr. Grubbs is hiding in one!
I found they dropped the most often from the Crypt Stalkers in Terrorweb Tunnel (4, 34) to (11, 24), but that could have been all luck. I finally found Mr. Grubbs in my 15th “Hidden Stash” which dropped off one of the Plaguebats along the Thondroril River.
Good luck, and watch out when you get him, he likes to jump around!