The mound builders were a different race of people than the natives that we stole this country from, but I'm not buying the horned skull story.
It's hard to tell from a picture, but the horned skull doesn't look right to me. Especially where the horn, or antler meet the skull.
It looks like a smooth transition, as if the protrusion is skull bone, antlers and horns are not like that. The very dark patina is probably hiding the artist's handiwork.
In the 1800's the world was even more of a mystery than it is now, many strange things were discovered with great fanfare, that later turned out to be hoaxes.