I've just read a bit about so called Welch Indians on wikipedia. Were there really Native American tribes in America that spoke some dialect of the Welsh language? It is said they are the progeny of a Welsh Prince called Madoc ab Owain Gwynedd. He supposedly travelled west to the North Atlantic and built a colony there. Is this true or is this false? My mind just blew, so I need some more facts about this.
/u/reedstilt mentioned this a while back in a most outrageous historical claims thread, so that should give you some idea of how much to credit such things.
This was a popular nineteenth-century red herring. For some time, the Celtic languages were thought to be linguistic isolates unrelated to Indo-Germanic - so called because it described a language group from India to the Germanic nations of Northern Europe. When Celtic was recognized as part of that group, it was subsequently called Indo-European.
Before that realization, however, people looked for affiliates for the Celtic languages. Stories about various Celtic travelers lost at sea and finding strange lands inspired many to look for possible North American connections. When people encountered Native American languages, many thought they were similar sounding to the Celtic languages, making Welsh-American Indian connections seem reasonable. The Mandans in particular were regarded as possible "Welsh Indians" in part because they were reputed to sometimes have fair skin and hair and they were seem to be more "civilized" than their neighbors. Others sought the Lost Tribes of Israel among Native Americans. Ultimately, these connections were proven to be outlandish, but in the context of the time they seemed reasonable. After all, Welsh "sounds strange" and so do Native American languages. And Welsh legend placed various people far across the ocean. Therefore, ... the imagination runs wild.