This pet is only available during Brewfest. (You obviously keep it after Brewfest, but it only spawns to tame during.) The only requirement is to get into the Grim Guzzler. They can be tamed as low as level 15, I believe. You also have to be drunk in order to see it, which is very easy during Brewfest (there’s even an alcohol vendor in the Guzzler), or you can get:
Note: It does not spawn in the Coren Direbrew Instance. So if you’re under the level where you can just walk through (40s or so), just spec into Camouflage, and Camo your way to the Guzzler. Or find someone with Direbrew’s Remote to help you, as it spawns a mole machine that ports you to the Grim Guzzler.
And do it without having to deal with the drunken effects after taming. (It spawns near the entrance to the guzzler and is smaller than you’d think. But still rather obvious if you’re drunk and see it lol)