Now on display in Paris in the Gallery of Paleontology and Comparative Anatomy (sadly, in a stairwell), it’s an excellent fossilized skeleton of a Palaeotherium Magnum:
Palaeotherium is an extinct genus of quadrupeds, most closely related to modern-day tapirs and horses. Based on the large number of their fossils that have been found to date, they widely roamed the forests that covered western Europe some 45 million years ago. But Palaeotherium browsed on low-hanging leaves (up to about 1-1/2 meters above the ground), leaving it at a disadvantage when Europe dried out, and the ancient forests were replaced by grasslands.
Most species of Palaeotherium were small, measuring about 75 cm (2-1/2 feet) tall at the shoulder. Palaeotherium Magnum was the largest of its kin, nearly reaching the size of a modern-day horse.