Potamotherium valetoni saint gerand le puy Musee d'Histoire Naturelle, Paris
The idea of Potamotherium being covered in fur is directly related to the fact that it is defined as a mammal. The mammalian nature of the animal can be seen in its head, including the teeth, and otter-like body. The head is extremely otter-like. The fact that early pinnipeds had similarly shaped heads and bodies is the only reason that Potamotherium has been confused for a pinniped in the past. However, the skull of Potamotherium is equally, and I assume more appropriately, mustelid-like and has caused the animal to be categorized as a mustelid. In terms of looking like an otter we know that the skull is very similar to that of extant otter species. The resemblance would have been noticeable, though differences would have certainly been recognized as well. However, at 1.5m (5ft) long it was a very elongate version of the otter. Short nasal chambers have led to the inference that sense of smell in this animal must have been fairly weak. The eyes and ears, though, appear to be highly capable.