Cossack (1844 – after 1862), also known as "The Cossack", was a British Thoroughbred racehorse and sire. In a career that lasted from July 1846 to June 1849 he ran twenty-two times and won three races. In 1847 he proved himself one of the best British colts of his generation, winning Derby and being narrowly beaten in the St Leger. Although he continued to run well in important staying races for the next five seasons, he failed to win another race and was retired to stud in after running once as an eight-year-old in 1852. No subsequent winner of the Epsom Derby has run beyond the age of five. He was later exported to stand as a stallion in France.

Cossack was a dark chestnut horse standing 15.2 hands high, described by the Farmer's Magazine as having a "neat blood-like head", "clean shoulders" and "very muscular quarters". He was sired by the Northumberland Plate winner Hetman Platoff out of the mare Joannina, a descendant of the influential broodmares Filagree, Web and Penelope.

Cossack was bred at Billing in Northamptonshire by R. C. Elwes. In 1845 the trainer John Day together with a Mr Dilly, visited Elwes stable to view his yearlings. Dilly was unimpressed by the small, upright chestnut but Day thought differently and, after some negotiation, bought him for 200 guineas on behalf of his patron T. H. Pedley, a bookmaker from Huddersfield. Cossack was sent to be trained at Day's stable at Danebury in Hampshire.

Cossack performed impressively in trial gallops at Danebury and had built up a considerable reputation before appearing on a racecourse. His first public run in July 1846 at Newmarket when he started favourite for the July Stakes. He ran poorly, however and finished only third to the filly Miami, who went on to win the following year's Epsom Oaks. Cossack did not race again in 1846.

At Newmarket's spring meeting, Cossack started 6/5 favourite and won the Newmarket Stakes, beating War Eagle by a length, with the pair pulling well clear and leaving the other runners in "a cloud of dust". At the same meeting, Cossack's stable companion, Conyngham, won the 2000 Guineas, and John Day found himself the trainer of the two leading Derby contenders.

The Derby was run on 19 May and Cossack started 5/1 second favourite behind Conyngham on 5/2 in a field of thirty-two runners, the largest ever assembled for the race. Even by the usual standards, the crowd was immense, with a "countless multitude" arriving via the newly opened Epsom railway station. After the police had cleared the spectators from the course, the race began with a clean start. Ridden by Sim Templeman, Cossack was second in the early stages, but moved into the lead by half way and soon opened up a clear advantage and was being cheered by the crowd as the likely winner half a mile from the finish. He was still three lengths in front in the straight and the only challenge came from War Eagle who moved almost level in the final furlong before being "shaken off". Cossack won easily by a length from War Eagle, with Van Tromp, who had been badly hampered early in the race, a further four lengths back in third. The value of the race was £5,250. At Royal Ascot in June, Cossack was allowed to walk over for the Swinley Stakes, when the other runners where withdrawn.

At Doncaster in September, Cossack ran in the St Leger. He started favourite at odds of 4/5 despite doubts about his stamina and the fact that Pedley "declared to win" with his other runner, Foreclosure. Templeman attempted to repeat his Epsom tactics and sent Cossack into the lead from the start. By the straight most of his rivals were struggling but Van Tromp, ridden by Job Marson made steady progress and moved alongside the favourite a furlong and a half from the finish. Marson sent his colt past the leader and Templeman, realising he was beaten, eased Cossack down in the closing stages to finish second, two lengths behind the winner but well clear of the rest. On 12 October, Cossack appeared at Newmarket for the two and a quarter-mile Cesarewitch Handicap for which he was set a weight of 118 pounds, including a six-pound weight penalty for finishing second in the St Leger. He finished fourth, behind the four-year-old Cawroush, who was carrying 98 pounds. His performance was creditable and reportedly led to speculation that he could have won the St Leger, had he not been ridden so aggressively.

Cossack's winnings for the season totaled £5,950, making him the second highest earner of the British season behind the Duke of Richmond's Red Hart.