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".

[1] He was sired by the Northumberland Plate winner Hetman Platoff[2] out of the mare Joannina, a descendant of the influential broodmares Filagree, Web and Penelope.

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,[5] a bookmaker from Huddersfield.

[5] 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.

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.

The race was run at a slow pace and Frank Butler, riding Cossack, found himself trapped behind a "ruck" of horses on the final turn.

Butler was forced to pull the horse to the wide outside and Cossack produced a strong finishing run, but failed by a "half neck" to catch Malton, a four-year-old to whom he was conceding thirty-one pounds.

[22] In July he ran for the third time in the Goodwood Cup and was narrowly defeated after his strong late run failed by a head against the filly Nancy, who was receiving twenty-four pounds according to weight-for-age rules.

[24] At Doncaster at the St Leger meeting in September, Cossack ran unplaced under top weight in the Great Yorkshire Handicap and was beaten the following day in a match race by the mare Maid of Masham.

Cossack's Derby win was seen as a rather fortunate one, owing a great deal to Templeman's enterprising tactics and the interference encountered by Van Tromp, who beat him decisively in all their subsequent meetings.

[5] Cossack retired to stand at Stanton, near Shifnal in Shropshire at a fee of 10 guineas for the 1853–1854 season[27] and was sold to Mr. Boythorpe and relocated to Derbyshire in 1855.