The Belfast Telegraph reports that 54 year old donkey named Rosie may be the oldest of her kind. Ted E. Bear is commonly cited as the oldest pony in the US, living to age 55. Back in March of 2006 I blogged about Jesse, a pony from Ireland who was 49. The original article about Jesse has since been removed from the Belfast Telegraph archives.
Guinness World Records
The Guinness Book of World Records is often cited as listing "Old Billy" a barge horse to be the record holder for the oldest horse at age 62, although I could not find the record on their website. The Guinness Book did record the oldest equine twins alive in 2007 were born in 1982 making them 25 years old at the time. This is significant because twin equines rarely survive much past birth. In 2007 Horse and Hound reported the demise of Sugar Puff, a 56 year old Shetland cross pony who made it into the Guinness Book records.
With our increasing knowledge of equine care and medicine, I have no doubt we will see these records broken in the future. Who knows, maybe you or I will own the world's oldest horse!