People take great pride in their names in Prydain. Gwydion is able to defeat the Horned King with knowledge of his true name. Achren is a threat to Arawn because she knows his. The fact that Taran does not have a name, that he is an unknown foundling of unknown parentage, is extremely troubling to him and he spends an entire book searching for answers. In the cosmology of the Prydian universe, a person's name is an extremely powerful and important thing.