Following is more speculation about the images of three separate fish the GCAS observed on their December 2019 field trip to the San Diego Mountain "Three-Fish" petroglyph site. Your faithful webmaster proposes that they are not fantasy images but instead are relatively factual representations of three separate fish species, at least two of which may have been marine fish - in other words, fish not local to the rivers and lakes of the desert Southwest but to Mexico's Gulf of California.
Up above there is the petroglyphic image of Fish Number One - it measures about 22 inches long by about 10 inches high. Note the rounded head, dorsal fins set far back, and the broad tail. I venture to guess that this petroglyph may be a fair and accurate representation of either a California opaleye or a Pacific porgy. The California opaleye (photo left, upper fish) inhabits coastal waters from California south along the Baja peninsula and into the Gulf of California and can reach a bit over two feet in length. The Pacific porgy (photo right) is found from Baja California and the Gulf of California to Peru. They also reach a maximum of two feet long.
Does any fisherperson out there have samples of other possible contenders?