Aatxe, also referred to as Aatxegorri—which translates as young red bull—is a shape-shifting spirit that most commonly appears in the form of a fiery red bull.
This spirit from Basque mythology is associated with the goddess Mari (who controls weather and seasons, and reigns over nature) and it’s sometimes believed to be a manifestation of her will or an enforcer of her dictates. Mari is said to be quick to punish those who lie or cheat, and the Aatxe is sent out against those people who have lied, cheated, stolen and have otherwise been disrespectful.
As Mari, Aatxe comes from the underground world of Euskal Herria. But he passes most of his time on the surface, haunting the caves and gorges of the Pyrenees Mountains, but coming out at night, especially during stormy weather, to attack and punish those who have gone against Mari’s precepts, but also to protect good and just people by making them stay home when there is danger.
Due to its shape-shifting abilities, Aatxe is capable of assuming many forms. Among the most common of which are that of a young man, an alpine goat, a pig, a horse, and sometimes a dragon. However, the defining appearance of Aatxe is that of a large and powerful red bull enveloped in fire.
As agent of the divine, one of an Aatxe's roles –besides punishment of the wicked— is that of a guardian spirit, especially over nature’s treasures, the domain of Mari. Hence, among those who really anger an Aatxe, are those who abuse nature, and those who don’t follow the sacred rituals.
Although not much of Basque mythology survived the arrival of Christianity –most of what we know, we learned from the study of places' names and the scant historical references of pagan rituals practiced by the Basques. Among those reference there's an old Basque witches’ song that says: “Yaun Gorril, Yaun Gorril,” which translates into English as “Lord Red, Lord Red,” and which it’s thought to make reference to the Aatxe, marking it as an important figure in Basque folklore. Furthermore, engravings and paintings depicting aurochs, bulls, and oxen –believed to be representations of Aaxte— have been found in many caves within the Pyrenees, such as Isturits, Sare, and Errenteria, implying that this Basque myth has its origins in the Paleolithic era.
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Matthews, J., Matthews, C. 2005. The Element Encyclopedia of Magical Creatures. Harper Collins.