In case anyone is curious, those are quills on the tails of the psittacosaurus and leptoceratops. They’re ancestral to feathers. Sorry, but the “feathered dinosaur” notion is largely inaccurate; the only dinosaurs that had feathers are those that are ancestral to birds. These are somewhat distantly related to those; you can see that archoceratops looks vaguely like a T-rex. Birds are related to the T-rex, but T-rex’s lineage probably lost quills shortly after diverging from birds’ lineage and probably never had feathers. I’m distinguishing between quills and feathers by defining a quill as a long, hollow tube of keratin protruding from the skin, and a feather as a highly branched quill, suitable for warmth if not necessarily flight.