Thunderhead was a chestnut horse, "built more as a sprinter than a stayer"[1] with a small white star, bred in France by Jean Desbons.
[4] On his first appearance as a three-year-old, Thunderhead was moved up in distance and won the Prix de Fontainebleau over 1600 metres at Longchamp Racecourse.
The field split into two groups on the wide, straight course, with Poincelet positioning Thunderhead behind the leaders on the stands-side (the left side of the course from the jockeys' viewpoint).
[6] Three weeks later, the colt attempted to win the French equivalent of the 2000 Guineas, the Poule d'Essai des Poulains over 1600 metres at Longchamp.
[9] In their book A Century of Champions, based on a modified version of the Timeform system, John Randall and Tony Morris rated Thunderhead an "inferior" winner of the 2000 Guineas.