1-Kentucky Derby Bred Horse: $16 million

The Green Monkey, a racehorse that was sold for $16 million in 2006, holds the record for most expensive pet. This colt, who was descended from two Kentucky Derby champions, set the quickest speed out of 154 horses offered for sale that year. This explains its exorbitant cost. Ironically, The Green Monkey retired in 2008 after two years and never competed in the Kentucky Derby.

This thoroughbred was injured when it fell during training, which prevented it from reaching its full potential. Only three races had been run with it by its owner, and the highest result was third. The Green Monkey was employed as a breeding stallion for prize racehorses after retirement. Its purchaser was able to recoup some of the $16 million investment as a result.