Pod is not a name for a group of hippos. Pod refers to a group of Cetacea. Cetacea are a widely distributed and diverse clade of aquatic mammals that today consists of the whales, dolphins, and porpoises. Cetaceans are carnivorous and finned. Most species live in the sea, some in rivers. The name is derived from the Latin cetus "whale" and Greek ketos "huge fish".
There are around 89 extant species, which are divided into two groups or parvorders, the Odontoceti or toothed whales, a group of more than 70 species that includes the dolphins and porpoises, and the Mysticeti or baleen whales, of which there are now 15 species. The extinct ancestors of modern whales are the Archaeoceti.