Definitely mammal. To me it looks like it comes from the Oligocene horse genus Mesohippus, or from Merycoidodon, the most common genus of oreodont mammal. The white bone and brown tooth preservation looks identical to mammal fossils found in the White River Formations of South Dakota, North Dakota, Wyoming, and Nebraska. Do know where the specimen is from?
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.
intresting about what you are saying with the stratigraphy. I have several oreodont jaw/skull fragments from the Brule Formation (another name for White River Fm.) and was labeled as Merycoidon. My oreodont specimens look typical of that genus, but I am not nearly as good at identifying mammal fossils as those of dinosaurs