Gandalf's Horse Shadowfax Dead -- 'Lord of the Rings' Star Dies ... After Being Euthanized

Gandalf's glorious steed Shadowfax from the "Lord of the Rings" flicks just took a permanent trip to the big glue shire in the sky -- after the horse got too sick and had to be put down.Shadowfax's owner Cynthia says the horse -- real name Blanco --…