Gandalf, the horse Mark Todd rode at last year's Olympic equestrian event in Hong Kong, has been put down after contracting a neurological disorder.
The 11-year-old horse recently suffered seizures and became blind in his right eye, becoming a danger to himself and his handlers.
Todd says it was a tough decision to make but the wellbeing of the horse was foremost.
He says it was also disppointing to lose a horse of Gandalf's calibre, particularly as he hadn't reached his best.
Todd's return to the 2008 Olympics came after eight years in retirement and despite Gandalf's demise, Todd still intends competing at London in 2012 and will compete at Badminton next year on Grass Valley.