Psidium, a chestnut horse with a narrow white blaze, standing 16.1 hands high,[1] was bred in Ireland by his owner Etti Plesch.

Psidium's dam, the Italian mare Dinarella finished fourth in the Oaks d'Italia and also produced the Poule d'Essai des Poulains winner Thymus.

In their book A Century of Champions, John Randall and Tony Morris rated Psidium a "poor" Derby winner.

[7] Timeform awarded Psidium a rating of 130, which is considered the minimum mark for an above average European Group One winner.

He was the leading British sire (in winning prize money) in 1966, thanks to the achievements of his son Sodium, which won the Irish Derby and the St Leger.