An Aatxe is an evil spirit in the shape of a red or fiery bull which, the Basques believe, leaves its cave on stormy nights to wreak havoc and destruction. It is also capable of assuming human form.
Aatxe is a Basque word, meaning a "young bull". It is also referred to as Aatxegorri which means "young red bull".
Basque folklore tells that the Aatxe originates from the underground world of Euskal Herria, and he is identified with the goddess Mari, possibly as a manifestation of her will. Mari is "quick to punish those that lie or cheat and generally the wrath of Aatxe is directed at those people who have lied, cheated, stolen or been generally disrespectful. There is an old Basque witches song that includes the line, 'Yaun Gorril, Yaun Gorril" which is translated as "Lord Red, Lord Red". It is most probably referrring to Aatxegorri and including him as an important entity in the pantheon and belief system of the pagan Basque."