Those were the teeth, by gum, of Parahippus cognatus, a Miocene ancestor of the modern horse. And they’re especially interesting because Parahippus and its descendants were grazers, rather than browsers, specializing in eating grasses. Do you see that in the teeth with their high crowns and their fancy surfaces?
No clear-cut winner, but GrizzlyD came closest in guessing Mesohippus. (And a tip of the riding hat to Steven Newton, who took the photograph.)