Olentzero is no ordinary old man that brings presents to kids on Christmas, he's a charcoal burner that is also a Jentil, a giant in the Basque mythology.

He was originally a pagan, but some say he stayed behind and converted to chrisitanism when he saw Jesus about to be born, while the other jentilak escaped. Other less favorable interpretations say he converted and betrayed the rest of the jentilak, showing the people their locations so they could be killed.

All these are modern interpretations for Christmas because afaik originally he was supposed to be somewhat evil.

Source: am basque