The first time I ever bought a Mystery Box I got the Twonicorn. It used up all the donuts I'd strained to harvest in those early levels while giving me a puking unicorn in return.
They never have the full set of a collection in the box. They'll bring out the last Fossil Fuel guy in another set of boxes.
And of course it acts like it's the last chance for some of these guys just to scare you. Not that these items AREN'T limited period to get you to rush buy and it could be a while, but it's still just to make you impulse click.