Cyllene (1895–1925) was a British Thoroughbred racehorse and sire. In a racing career that lasted from 1897 until 1899, Cyllene won nine of his eleven starts, winning major races and being regarded as the best horse of his generation at two, three and four years of age. In a stud career which began in Britain and ended in Argentina, Cyllene became an important and influential stallion. He sired four winners of The Derby and is the direct male-line ancestor of most modern thoroughbreds.

Cyllene, a chestnut horse with a white stripe and two white socks was bred by his owner, Charles Day Rose at the Hardwicke Stud near Pangbourne in Berkshire. He was a small, late foal, born in May and was never entered in the Classics as he was thought unlikely to be strong or mature enough to compete at the highest level at an early age. Cyllene was trained throughout his racing career by William Jarvis at Waterwitch House stable at Newmarket, Suffolk. Rose thought so highly of Cyllene that he arranged a "luxurious" private train to ensure that the colt returned to Newmarket promptly after his races. Cyllene's most regular rider was the 1899 Champion Jockey Sam Loates.

Cyllene's sire, Bona Vista was a high class racehorse who won the 2000 Guineas in 1892. He was considered a failure as a stallion and in the year of Cyllene's birth he was sold and exported to Hungary. Arcadia, Cyllene's dam, won two races and was a half sister to Van Dieman's Land, who ran third in the Derby.

In the spring of 1897, Cyllene won small races at Liverpool and Gatwick "in good style". He was then sent to Ascot for the Triennial Stakes where he defeated Nun Nicer, a filly who won the following year's 1000 Guineas, establishing himself as one of the best of an apparently "above the average" generation. At Sandown in July he won the National Breeders' Produce Stakes by a head to take one of the season's most valuable two-year-old prizes, despite being unable to obtain a clear run until the closing stages.

After a break of three months he returned to the racecourse in October for the Imperial Produce Stakes at Kempton Park Racecourse in which he narrowly failed to give ten pounds to Dieudonne, a colt who went on to win the Middle Park Stakes later in the month.

As he was ineligible to run in the Classics, Cyllene's three-year-old campaign was somewhat restricted. He was disappointing on his debut, finishing third at odds of 2/11 in the Column Produce Stakes at Newmarket in April but was never beaten again. In May he easily won the Newmarket Stakes by four lengths in record time, and was then rested until autumn. Before he returned, the form of his Newmarket win was boosted when Jeddah, who had finished fifth in the race, won The Derby.

In September he produced his best performance to date to win the Jockey Club Stakes at Newmarket by six lengths from a field which included Dieudonne and the Classic winners Airs and Graces and Chelandry. On his final start of the year he won the Sandown Park Foal Stakes by four lengths, taking his winnings for the season to £14,563.

Cyllene's four-year-old season was compressed into a three-day period in June at Royal Ascot. At the start of the meeting he won the Triennial Stakes which served as a warm-up for the Ascot Gold Cup two days later. Racing over two and a half miles in the Gold Cup, Cyllene turned the race into a "procession", taking the lead two furlongs out and pulling clear to win by eight lengths from Lord Edward and the Prix du Jockey Club winner Gardefeu. According to one correspondent, Cyllene "simply played" with the top quality field, with his jockey Sam Loates having time to stroke the horse's neck and touch his cap to the cheering crowd in the closing stages.