Colin (1905 – 1932) was an undefeated champion American Thoroughbred racehorse who won all 15 of his races, including the 1908 Belmont Stakes. He was the 1907 and 1908 Horse of the Year as well as the 1907 Champion Two-Year-Old Male and 1908 Champion 3-Year-Old Male and was inducted into the National Museum of Racing and Hall of Fame.

In 1907, he swept the major two-year-old stakes races including the Belmont Futurity and Champagne Stakes and was the consensus Horse of the Year. His three-year-old campaign was cut short by injury but he was still Horse of the Year based on his three wins including the Belmont Stakes. As a sire, he suffered from fertility problems but still sired multiple stakes winners.

Colin was inducted into the National Museum of Racing and Hall of Fame in 1956. In the Blood-Horse magazine List of the Top 100 Racehorses of the 20th Century, he was ranked number 15.

Colin was a brown stallion with one white sock, one pastern, one partial fetlock, and a blaze and snip on his face. He was foaled in 1905 at Castleton Stud in Kentucky and was owned by London-born financier James R. Keene. Colin was from the third crop of foals by the stakes winner and leading sire Commando (by Domino), who had been bred by James Keene. Colin's dam was the English stakes-winning Pastorella (GB), by Springfield. She had been imported to the US by Marcus Daly and was purchased by Keene in 1901.

Colin was trained by Hall of Fame inductee James G. Rowe Sr. Rowe had handled many top horses in his long career, including Sysonby, Hindoo (who was never unplaced), and the first filly to win the Kentucky Derby, Regret. Rowe and his horses Miss Woodford, Luke Blackburn, Whisk Broom II, Commando, and Peter Pan were inducted into the Hall of Fame.

James Keene was not initially enthusiastic about Colin, noting his disfiguring curb, or thoroughpin, meaning that the colt had an enlarged hock. He'd been just as disdainful of an earlier purchase: Colin's grandsire Domino, (another eventual Horse of the Year in 1893 and Hall of Famer), but his son, Foxhall Keene, bought Domino anyway.

A friend of Keene's, De Courcey Forbes, always named the Castleton foals. Colin was for "Poor Colin", a pastoral poem by the English poet laureate Nicholas Rowe, thus connecting the name of Colin's dam and the name of his trainer, who took a keen interest in his horses. A hands-on trainer, Rowe was famous for the personal attention he paid to his horses. He literally traveled in the same railroad car with them. Aware that Colin's swollen hock would give him trouble, Rowe attended to it with massages and cold water baths.

Consistently rated as one of the best horses in American racing history, and a celebrity with both fans and horsemen, Colin started fifteen times in his two-year career and never lost. Twelve of these races came when he was a two-year-old. In an age that valued stamina and maturity, Colin was still viewed with awe by the horsemen of his time. Sportswriter Abram Hewitt said, "The blood surges, and the pulses quicken at the very sight of such Olympians on the track." Hewitt had "listened to old-time horsemen talk about Colin with an other-world expression on their faces." Colin would earn National Champion honors as the American Horse of the Year for 1907 and 1908. He was also America's Champion Three-Year-Old Male in 1908.