Today's pet is one of the upper crust of Marleybone (definitely worthy of a prince or madame title)!
Yes, the Rat Magician is one of the mighty morphin Marleybone pets, available rarely at the bazaar (as with most droppable pets), but ultimately found hidden in Sprocket's loot table clear at the top of Counterweight East.
Interesting that they drew the hat separately from the rat magician, isn't it? They must have wanted to make sure to get some detail right on the forehead of that rat. hmmm.
Here's a little tip for farming your Rat Magician: Once you're at the top of Counterweight East you can respawn Sprockets over and over and over again by running down the ramp and back up again. He'll just keep coming back! THAT'S FARMTASTIC FUN!