Answer:
Mandan (like almost all modern native American tribal names) is not the real name of the tribe, who originally called themselves Numakaki or (later) Ruptare, meaning "men" or "people". The word Mandan is a French misunderstanding of the name used for the tribe by the Assinibiones.
The Mandans are part of the Siouan language family but the Mandan language has long been influenced by neighbouring Hidatsa (another Siouan language). They constructed semi-permanent villages of earth lodges along the banks of the Missouri river and its tributaries the Heart River and Knife River in North and South Dakota. They used "bull-boats" to cross the river - these are superficially similar to Welsh coracles but are not connected in any way. They fished and planted crops, occasionally hunting buffalo and trading with the Crows and other Plains tribes.
The Mandan tribe was fatally reduced by warfare, smallpox and other epidemics during the 1830s and the remainder joined the Arikaras and Hidatsas; the last full-blood Mndan died in 1971 so apart from mixed-blood descendants the tribe is considered to be extinct.
The Mandans were made famous in the early 1830s by the dreadful and unscientific misreporting of the explorer and artist George Catlin, who was more than partially deaf; he had already been aware of the supposed medieval voyage of the Welsh prince Madoc and a mythical tribe of "Welsh Indians" - his assessment of the Mandans made a number of serious mistakes and distorted the evidence to make them appear to be the tribe in question.
Catlin saw that many of the Mandans had fair skin, fair hair and blue eyes, but he ignored the fact that their neighbours the Hidatsas and Arikaras also included many people with the same racial characteristics - he was also unaware that it is a purely native genetic feature and nothing to do with Welsh Princes.
Catlin, being hard of hearing, mis-heard many Mandan words and connected them with Welsh words; in fact Mandan is purely a Siouan language with no Welsh elements of any kind. When challenged on this point he made up an unconvincing story that the Mandans had a "secret language" only known to himself . . . complete nonsense.
Catlin also pointed to the remains of similar earth-lodge villages along the entire course of the Mississippi-Missouri, claiming that this proved that Madoc had landed in the vicinity of modern New Orleans. In fact these villages belonged to many other tribes; the Mandans originally lived in Ohio and Wisconsin before moving to the Dakotas.
The myth of Prince Madoc establishing a tribe of Welsh Indians in North America is full of historic inaccuracies and does not stand up to careful scrutiny: 12th century Wales was not known as a sea-faring nation and had no ships capable of making a transatlantic voyage; there is no historic or documentary basis for the existence of Prince Madoc and later Welsh explorers found no evidence of Welsh language in use among the Mandans.
The story of the Mandan tribe is one of a proud and unique native group, with a rich religion and history of their own. Its end is ultimately tragic, with the loss of a great deal of native heritage. Forget the Madoc myth and study the Mandans as a native American people who deserved a better fate.