This is Brontotherium (I'm not entirely sure which species...) at the Peabody Museum in New Haven, Connecticut. Doesn't the mount just ooze raw power? There's so much movement conveyed with such a fixed, inanimate object.
I wish I had mad Photoshop skillz so I could take out the glare and all that. Really, I wish I had Photoshop, period.
For some reason, I remember the display saying that the animal was in fact Brontops, though I could be wrong. I'm not 100% sure it's Brontotherium, but if EvoWiki is correct, the "slingshot-shaped" horns strongly suggest that it is.