In particular, the many tales of Welsh-speaking natives in the Americas led to informed (or uninformed) speculation from Early Modern times up to the present about the fate of Madoc and his crewmates.
This particular page will, however, deal with a supposed description of the lost Madocke written by the Dutch author of Van den Vos Reynaerde, who identifies himself in that latter work as Willem die Madocke maecte ("Willem who made, i.e. wrote, Madoc").
Whilst regarded as having been lost in the centuries after Willem, British author Donald McCormick, writing as Richard Deacon, claimed to have received a description of a text in a 17th century French manuscript curated by a certain Edouard Duvivier of Poitiers.
Though Gwyn A. Williams accepted much of Deacon's work relatively uncritically, further research has more or less proven that Deacon's claims are false. Nevertheless, there seems to be little in Deacon's deccription of Duvivier's alleged correspondence which is particularly outlandish: many of the motifs can be found in other sources of a similar date: indeed, the Belgian scholar Louis Peeters noted many similarities with the adventures of the title character with the mariner Mador in the Histoire de Fouke Warin.
"Guillaume qui fait Renaud" was familiar with Wales. He visited the country as part of a group of Flemish mercenaries in the pay of the English king, only to defect. His fascination with all things Welsh led Guillaume to "bards and men of the sea," who introduced him to the story of the great adventurer Madoc, and it is probably that Willem dwelt for a time on Ely, or Lundy, an island which had connections of the reputed navigator.
Madoc was a nobleman with the sea in his blood: his grandfather was "half a Viking." Coming of age, Madoc, in the guise of a monk, visited the court of the French king Louis VII as a representative of the Welsh of Gwynedd. At some point, he came into contact with his romantic interest, described as a river-nymph, who goaded Madoc (himself given the description of a "mer-man") and his companions, a group of bards, into seeking the Fountain of Life. Bizarrely, the river-nymph's otherworldly heritage is revealed by her wearing fishing nets on her legs.
Whilst early modern writers, such as John Dee, claimed that Madoc's travels took him to the North American mainland, it must be noted that McCormick (or, if you prefer, Duvivier or Willem) never explicitly claims that Madoc reached such a vast landmass: indeed, the idea of travel to terrestrial paradise islands and great septentrional maelstroms seems to be more at home in the realm of medieval voyages, such as that of St. Brendan, than learned speculation about the New World, such as that evinced in Niccolò Zen the Younger's tales of Zichmni and Estotiland.
See Cor Hendriks' Richard Deacon, master of disinformation (2016) for an excellent overview of "Deacon" McCormick and his work. There is no way I would've been able to produce this page without reference to Hendriks' article.