His owner, Cynthia Royal, writes to The One Ring website that Blanco "suddenly and without warning fell critically ill." He suffered "extremely serious issues with his intestinal track and liver," and Royal says she was forced to put him down after he failed to improve earlier this week.
Blanco first appeared as Shadowfax, the wizard Gandalf's (Ian McKellen) companion and mount, in The Lord of the Rings: The Two Towers. Gandalf calls Shadowfax "the lord of all horses" -- the king of a sentient breed of wild horses called mearas.
"While this leg of Blanco's journey has passed and my heart still aches, I will live in peace knowing we gave our all to aid in his recovery," Royal concludes.
[The One Ring]