Propalaeotherium was named by Paul Gervais. Its name means "before Palaeotherium". It was considered a member of Palaeotheriidae by Hooker (1986).
A 2004 study found it to be an equid instead. When the show was aired, Propaleotherium was assumed to be one of the ancestors of the modern horse, and had been depicted as such. Now, however, it appears that it belonged to a different branch of equids.
The species P. parvulum and P. messelensis have been assigned to the equid genus Eurohippus.
In Walking with... Series
In the show, Propalaeotherium had four hoof-like toes on its foot and was the size of a modern cat, or some of the other ancient mammals of Paleocene and Eocene epochs. Being so small and vulnerable Propaleotherium was prey for the giant flightless bird Gastornis and the primitive walking whale Ambulocetus as shown in Walking with Beasts. If attacked, Propalaeotherium could only run and hide in the jungle undergrowth - it had no other natural defenses, and sometimes (as shown in WWB), Propalaeotherium wasn't cautious enough, and got captured and eaten instead.
In addition, at the end of the episode, the audience can see another dead Propalaeotherium that died during the gas eruption of the Messel. Tacitly, this also indicates the eventual fate of this entire species: it died out completely without leaving any descendants, as the global jungles vanished at the end of the Eocene, and new, bigger and tougher herbivorous mammals appeared on the planet.