The Great Horned Serpent of Native North America:
Below, Water Panther or Mizhipizhw. Below that, Powdr Horn Plesiosaur from 1700s and the Petroglyph again. I do not consoider the Water Panther to be a Plesiosaur but to be akin to the Irish Master-Otter, for which this is a good representation (Ignoring the antlers)
Below, Western (Great Plains) subtype of Great Horned Serpent: in both this case and in the case of the Water Panther above, any sightings of creatures with such antlers in the water would actually be swimming deer or moose. However Scott is implying that other than that,the water monster represents a Plesiosaur, and compares the artwork depicting the creatures to the reconstructions of Plesiosaurs on the Right.