The name Millan is believed to have originated from the Gaelic language of Ireland and Scotland. It is derived from the word "mìl," which means "gentle" or "kind," and "an," meaning "one." The name was likely first used in the early medieval period, around the 5th to 7th centuries AD, when Gaelic culture and language were prevalent in these regions.

One of the earliest recorded instances of the name Millan can be found in the life of Saint Millan (472-570 AD), a Galician monk and hermit who is venerated in Spain. He is credited with founding several monasteries in the region and is the patron saint of Treviño, a town in the Basque Country.

Another notable figure with the name Millan was Millan the Younger (c. 625-710 AD), a Galician monk and disciple of Saint Fructuosus. He is recognized for his role in the establishment of the Monastery of Samos, one of the most important monastic centers in medieval Galicia.

In the 12th century, Millan de la Cogolla (c. 1035-1111) was a Castilian nobleman and knight who became a monk and is known for his contributions to the development of the Romanesque architecture of the Kingdom of Navarre.

During the 16th century, Millan Astray (1879-1954) was a Spanish military officer and the founder of the Spanish Foreign Legion. He played a significant role in the Rif War and the Spanish Civil War, and his name is closely associated with the Legion's motto, "Valour has no age."

Millan Lubin (1929-2012) was a Swedish actor and director who had a prolific career in both film and theatre. He appeared in several acclaimed Swedish films and directed numerous plays, earning recognition for his contributions to the performing arts in Sweden.

While the name Millan has its roots in Gaelic culture, it has been adopted and used in various parts of the world, particularly in Spanish-speaking countries due to the influence of Saint Millan and other historical figures bearing the name.