Viserion is named for Daenerys's brother Viserys, also called the Beggar King. Daenerys named him this to give Viserys the honor in death that he never possessed in life, as he was cruel and abusive, but still a Targaryen. One can also assume most of it has a great deal to do with his color, white and cream, the color of Viserys's hair and skin, although it is common in the Valyrian coloring of the Targaryens.