This is Grunkk. I got him from a Menagerist Goblin in Season 12. The flavor text for this pet says: The way he breaks buildings and other structures is just so cute.
The Menagerist Goblin is the only type of Goblin that drops pets. They also drop some random loot. There is no way to know what pet a Menagerist Goblin will drop until after you kill him.
The pets dropped by the Menagerist Goblin look like a scroll.
The Grunkk item goes into your bag when you pick it up. The flavor text on the item says: Is that a miniature demon?
Players can find all of their pets in their Wardrobe. All of your characters can equip any of the pets that are in there.
Grunkk is a large pet. In the screenshot above, Grunkk is running next to my Treasure Goblin and both are following my Barbarian. Grunkk has two arms and four legs. He is wearing a little pair of either underwear or perhaps a diaper.
Grunkk appears to be a baby demon. I think he looks a lot like a little Siegebreaker Assault Beast.
There stands Gurr the Barbarian with Grunkk the “miniature” demon.