Combinations of different animals. The "dinosaurs" on the Narmer Palette, for instance, are actually called serpopards -- a cross between a serpent and a leopard. The long necks could also just as easily have been based on the giraffe.
Don't forget that ancient art is full of depictions of mashups of different animals (or even humans and animals). Mermaids, centaurs and so on are just a couple of examples that come to mind.
Even in the most convincing cases, the evidence for humans and dinosaurs coexisting is very, very, very, very weak. And that has to face off against hundreds of thousands of high-precision, rigorously vetted and cross-checked radiometric and other similar measurements.