I think Evan is right on. Certainly looks like White River preservation to me. To narrow it down, it is the lower right jaw with the first and second molars of an oreodont, not Mesohippus. Without knowing where in the White River stratigraphy it came from and with no premolars present it’s hard to say which oreodont species it is, but like Evan said it could be Merycoidodon sp. or possibly Miniochoerus sp.