Formosa (1865–1881) was a British Thoroughbred racehorse that was the first winner of the English Fillies Triple Crown in addition to running a dead heat with the colt Moslem for the 2,000 Guineas Stakes.

[2] She was a dark chestnut, well-muscled filly, that stood 15.1 hands high and was noted by the British Farmer's Magazine to have "a pleasing head", a "strong, slightly crested neck" and "unexceptionable legs, upon which she stands as firm as a rock.

At the July meeting at Newmarket, Formosa won the Chesterfield Stakes at 20 to 1 odds, beating the filly Léonie, the colt Suffolk and the betting favourite Athena.

[13] On 28 April, Formosa ran against thirteen opponents in the 2,000 Guineas Stakes over the Rowley mile course at Newmarket where she disputed favouritism with Green Sleeves.

Moslem held a slight advantage for most of the way, but George Fordham moved Formosa up to challenge him in the final furlong and the two horses crossed the line together in what was described as "one of the finest struggles" in the race's history.

The judge declared a dead heat, meaning that the two horses would run again over the same course unless the owners agreed to divide the prize money.

The deciding heat was almost immediately scheduled for the end of the day's racing but half an hour later it was announced that the owners had agreed to divide the stakes.

She was amongst the leaders from the start before drawing clear inside the final furlong to win very easily[14] by three lengths from Athena and Lady Coventry.

Formosa, ridden by Fordham held on to a three-length lead into the straight over Virtue, Léonie and Curfew Bell, who faded and dropped back.

[16] On 9 June at Ascot, Formosa finished sixth in the one mile and five furlong Prince of Wales's Stakes, the race being won by Baron Rothschild's colt King Alfred.

[18] At Doncaster on 9 September, Formosa contested the St. Leger Stakes against eleven other horses, including previous rivals Virtue and King Alfred as well as Mr. Hodgman's Chester Cup winner Paul Jones.

[24] A few days later, Formosa finished third in the Ascot Gold Cup, losing to the three-year-old filly Brigantine (the 1869 Oaks winner and also sired by Buccaneer) and Blue Gown.

[29] At the September Doncaster meeting, Formosa was fourth in the Great Yorkshire Handicap, losing to the colts Géant des Batailles, Argyle and Paul Jones.

[35] The Sporting Review commented on Formosa's poor form during the season, "[she] is being hacked about all over the country and losing her reputation by being beaten for paltry prizes over cup courses when she has shown again and again that like all the Buccaneers she is not a genuine stayer.

[10] Formosa was bought in August 1876 by Thomas Gee of Dewhurst Lodge for 2,700 guineas at the Newmarket yearling sales with her filly foal sired by Winslow,[37] later named White Poppy.