A tiny mammal by our standards, Propalaeotherium was ironically actually rather large for its day, as the mean average size of mammals for its time as the size of a small shrew. Indeed, even today, this mean average is still only around rat sized. However, we have Elephant sized mammals, whereas during the day of Propalaeotherium, no such great megafaunal mammals existed at all. Propalaeotherium, being so ancient in the linneage of the Equines, hadn't even evolved hoofs. Instead, they had clawed 'hoof-like' toes. This animal would have been heavily reliant on acue sense to protect itself in the dangerous and weird forests of the Early Eocene. Giant Terror Bird, Gastornis, would have been a common predator of Propalaeotherium; needing to ambush such skittish and agile little horse ancestors.