Rossitor and Aglow (1973)
The highest-profile equine mix-up since the introduction of equine passports in the late 1960s occurred between the fillies Rossitor and Aglow, both bred in 1970 by Someries Stud and sent into training with Gordon Smyth. In 1973 Rossitor won twice as Aglow, and Aglow ran as Rossitor without winning.
The mix-up was not discovered until 1982, when Rossitor's son Kalaglow was about to win the Eclipse and King George. If his correct parentage had been known from the start, he would presumably have been called Kalator.
And more recently. . .
Ruby Queen: Mahoning Valley, Ohio, US (November 4, 2015)
Leathers Slappin, a four-year-old gelding, ran as Ruby Queen, a three-year-old filly from the same yard of owner-trainer Shane Spiess, in a race for fillies and mares. He won at 110-1.
The Ohio commission later ruled there was no evidence of any attempt at fraud. Thomas Clark, a horse identifier, was fined $500 and suspended for 60 days. Spiess, who was not at the track, fined $500 and suspended for 30 days.
Magic Feet, Kenilworth, South Africa (November 6, 2013)
The 66-1 maiden winner was actually Ignition Lady, another three-year-old from the same yard of Darryl Hodgson. In this case the horse identifier, Nasaphi Sankahla, lost her job. 'Magic Feet' was disqualified.