"Pet collectors will also be thrilled to befriend a new cosmetic pet—Rocky is a regular chip off the old block, and he’ll be happy to rumble along and pick up your gold as you adventure through Sanctuary!"(src)
In-game, he will collect gold for its master. Intentionally or not, this earth elemental creature was included together with Might of the Earth set from Haedric's Gift and the earth-themed Season portrait.
He's made of stone, he's smelly, and he's all yours!