Well, dog, that is a good question. Since I am not a learned person on the subject of early Spanish explorers and their religious practices, I cannot say with any certainty that they used devil / demon likenesses. BUT, I have a hard time believing that they actually WOULD. Consider the time of the Inquisitions, not to mention the normal fears and superstitions felt by, not only the Spanish Conquistadors, but the Mexican peoples, too. Their Catholic based religious practices are filled with icons for every saint imaginable, in addition to the ones for Jesus and Mary. Would the ACTUALLY construct icons for the devil or demons........even in shadow forms or carvings on the trails to their "workd sites"? We are told, by the experienced researchers, that each expedition was accompanied by a Priest or Padre. Would THEY have condoned the use of any likenesses representing the devil or demons?? Good question. They did allow and , sometimes even instigated the mistreatment of the indigenous peoples they encountered in Mexico and the various sections of what is not the US. In some cases, even allowing the execution of some for some perceived heretical reasons.
I wonder if some of the horned, shadow figures could represent another group of people.........such as the Vikings or the groups from the MeseoAmerica area. Doesn't some of the masks used by the Aztecs, Incas, and Mayans have horns carved as part of the headress portions?? The native American tribes of North America had some Medicine Men / Shamans who used headresses with cows' horns attached.
There is a wide area of history that needs filling in by someone who is not afraid to buck the "conventional wisdom" of the main stream science community. There are a few "out there", but not enough........yet.
I wasn't going to mark anything on these last 2 photos, but I couldn't resist for long concerning the second one. I connected the 2 circles because the shadow figures look so much alike in the different areas. That's a long legged bear on the left edge, in the square.....with what might be some 20th century spraypaint graffitti above it in that other square....."6, 7, 9" ?? The other 3 squares enclose some interesting things to look closer at for clarity. The lower one in the center of the pic, seems to have a recessed area with a rock "head" hiding a cavity?? In the upper, center left square are what looks like several large carved letters. The right square (upper center) has a black triangle, a black tombstone (or shield) shape and a carved, long faced head. The square in the upper right section has a long, faced head carving.
There's several other things, but to mark anything else would just clutter up the pic.
EDIT: I forgot to mention the square just to the right side of the bear. There are some shapes that look like raised, carved figures.