In the second half of the 19th century, historical-legendary literature appeared defending the preservation of the fueros (charters of local autonomy) and constructing a Basque identity. These legends were mostly invented by the writers themselves. The most notorious among them, gaining widespread credence, was the myth of Aitor, the purported patriarch of the Basque race, but in fact an invention of the French-Basque journalist Augustín Chaho (1811–1858). It was only from the end of the 19th century that the myths and legends told by the Basque rural population were properly collected and recorded. Some Basque legends were collected by foreigners and French-Basques: Légends et récits populaires du Pays basque (1875) by Jean François Cerquand (1816–1888), Basque legends (1879) by Wentworth Webster (1828–1907) and Le folklore du Pays basque (1883) by Julien Vinson (1843–1926). The most significant contribution to research on Basque mythology from within Spain was Euskalerriaren yakintza: Literatura popular del País Vasco (1935-47), a collection of myths and legends by José Miguel de Barandiarán (1889–1991) and Resurrección María de Azkue (1864–1951).

Barandiarán attempted to establish a relation between pre-historic magical arts, the cult of Mari’s animal avatars (Mari was one of the most important Basque mythological figures) and modern Basque mythology. In this way, a continuity of the Basque race or tipo vasco (Basque type) was established, feeding at the same time the idea that the Basques had primordial roots. He founded the yearbook Anuario de Eusko folklore (“Basque folklore yearbook”, 1921-34; publication resumed in 1955), and also published his research in his Paleografía vasca (1921), El hombre primitivo en el País Vasco (1934) and Mitología vasca (1960).

The supposed Basque primitive religion was reconstructed by Barandiarán with the help of archeological findings such as the bones of a cave bear in the French-Basque Labourd region. These bones were considered to be sacrificial remains and thus proof that a Basque primitive religion could be traced back to ancient times. Influenced by Wilhelm Schmidt’s (1868–1954) theory of primitive monotheism (Der Ursprung der Gottesidee, 1912-54), Barandiarán (a priest, like Schmidt) hoped to provide proof for a Basque primitive monotheism which was also propounded in other cultural fields, such as historical-legendary literature and history-writing.