One of the current debates in mammalian paleontology is the origin of the Cetaceans (whales, dolphins, and porpoises). It is generally accepted that cetaceans are mammals which through evolution have 'returned to the sea'. Evidence of a land ancestry comes not only from the fact mammals and mammal like reptiles first appear as land dwelling creatures, but also from the useless vestigial pelvic and thigh bones present in most modern whales. Earlier fully marine cetaceans such as Basilosaurus actually had tiny rear legs which are much too small to be of any practical land dwelling use, and therefore a remnant from their land based ancestor. Several intermediate forms between land ancestors and fully marine modern cetaceans have been discovered, but what the land dwelling ancestors of cetaceans were is still a matter of debate. Based on similar tooth morphology, it was first theorized that this ancestor was a type of Mesonychid, but molecular studies suggest that Hippopotamids are genetically close to modern whales.  

The idea that cetaceans are descendant from a hoofed mammal comes from molecular analysis. When a map is made comparing the molecules in the genes of modern animals, cetaceans are mapped closest to the Artiodactyls or  even toed ungulates. The connection between cetaceans and Mesonychids also be made when comparing anatomy. Both Mesonychids and early cetaceans have similar triangular shaped teeth with a prominent middle cusp or point and two relatively equal sized cusps on either side. Other similarities include a loss of a collarbone, similar holes for arteries in the base of the skull, and specialized upper arm bones. Mesonychids are also the only carnivorous ungulates, and although many are filter feeders of tiny organism, no cetacean is a plant eater. 

The transition from Mesonychid to cetacean is also easy to document from fossil evidence. Mesonychids were often shore dwelling animals that hunted both on land and in the shallows, and it is not hard to imagine a shore dwelling creature becoming more specialized and eventually 'returning' to the ocean (similar to modern seals and walruses). Intermediate forms such as Ambulocetus and Pakicetus have been found, and these animals closely resemble Mesonychids with their fully functioning legs and similar tooth morphology, but with striking differences. Pakicetus has a similar body design, but a head more closely resembling archaic cetaceans. Ambulocetus is similar in design to Pakicetus, with the addition of flippered feet, and most likely moved better in the water than on land like a modern otter or seal. Beyond Ambulocetus, it is easy to trace the ancestry directly to modern cetaceans.

With the above evidence, why are Hippopotamids or rather a hippopotamus like creature considered as a likely candidate? The theory arises from where the earlier mentioned genetic mapping places cetaceans. Of all the modern artiodactyls, cetaceans appear closest to modern pigs and hippos. This could possibly mean a hippo or hippo like creature is a better candidate for land ancestry, but the similarity is from genetic analysis alone. The argument against this may be that Hippopotamids are an early side branch of the artiodactyls, diverging from the ancestral line at about the same time as the Mesonychids. Hippopotamus also have a semi aquatic lifestyle, even in modern times, so it is not difficult to imagine a similar evolutionary history of increasing specialization. The fossil evidence, however, seems to point more towards a Mesonychid ancestry. 

More compelling evidence may also be the fact that although there are modern Hippopotamids, there are no modern equivalents to the carnivorous Mesonychids. It may be that as the hoofed carnivores were driven to extinction by competition from the true carnivores such as canids and felids, only those specializing in marine adaptations survived.