The suborder Suina (also known as Suiformes) is a lineage of omnivorous non-ruminant artiodactyl mammals that includes the pigs and peccaries of the families Suidae and Tayassuidae and their fossil kin. Hippopotamidae has historically been classified among the Suina for morphological reasons, but is now more often classified as the sister group of the whales, or Cetacea.
The Merycoidodonts, or "oreodonts," a branch of the tylopoda, were often considered suines due to the popular, though inaccurate, description of them as "ruminating hogs". Oreodonts were not suines, they were more closely related to camels. Similarly, the entelodonts, had long been classified as members of the Suina. Spaulding et al. have found them to be closer to whales, than to pigs in his Cetacodontamorpha.
Some morphological studies have suggested that the hippopotamus family Hippopotamidae was part of the Suina, but a growing body of morphological and genetic evidence has suggested that they share a common ancestor not with the Suina, but with Cetaceans—the clade that includes whales and dolphins. Whales and artiodactyls form a clade called Cetartiodactyla.
The most recent research into the origins of hippopotamidae suggests that hippos and whales shared a common semi-aquatic ancestor that branched off from other Artiodactyls around  Descendants of this hypothesized ancestor likely split into two branches around . One branch would evolve into cetaceans, possibly beginning with the proto-whale Pakicetus from and other early whale ancestors, known as Archaeoceti, which eventually underwent aquatic adaptation into the almost completely aquatic cetaceans..
The anatomy of the Suina differs from other even-toed ungulates. For example, they have maxillary (upper) teeth in front, which allow for the proper chewing of food. In contrast, other even-toed ungulates, such as goats and deer, have front teeth only on the bottom; this doesn't let them chew very well, thus they swallow and regurgitate their food to allow rumination.
Most even-toed ungulates have a four-chambered stomach. In contrast, the Suina have a simple stomach that allows an omnivorous diet.
Most members of Suina have toes rather than hooves. While most artiodactyls have long slender legs, the Suina generally have short, stubby legs.
- Spaulding, M; O'Leary, MA; Gatesy, J (2009). "Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution". In Farke, Andrew Allen. PLoS ONE 4 (9): e7062. doi:10.1371/journal.pone.0007062. PMC 2740860. PMID 19774069.
- Boisserie, Jean-Renaud; Fabrice Lihoreau and Michel Brunet (February 2005). "The position of Hippopotamidae within Cetartiodactyla". Proceedings of the National Academy of Sciences 102 (5): 1537–1541. doi:10.1073/pnas.0409518102. PMC 547867. PMID 15677331. Retrieved 2007-06-09.
- Gatesy, J. (1 May 1997). "More DNA support for a Cetacea/Hippopotamidae clade: the blood-clotting protein gene gamma-fibrinogen". Molecular Biology and Evolution 14 (5): 537–543. doi:10.1093/oxfordjournals.molbev.a025790. PMID 9159931.
- Ursing,B.M.; U. Arnason (1998). "Analyses of mitochondrial genomes strongly support a hippopotamus-whale clade". Proceedings of the Royal Society 265 (1412): 2251–5. doi:10.1098/rspb.1998.0567. PMC 1689531. PMID 9881471.