The history of the Sforza monument can be divided into two periods, distinguished by two projects that differ in terms of shape and size, but share a substantially identical casting technique.
The first project: a rider on a prancing horse
The first project, probably completed before Leonardo was commissioned to create the monument, must have featured a life-size rearing horse. Several studies of rearing horses dating around the mid-1480s, collected today in the Royal Library at Windsor and other museums, suggest Leonardo's interest in this topic but there is no documentation about his involvement before 1489.
Although there is a beautiful Leonardo drawing in the Royal Collection at Windsor (12349r-v) that reproduces a foundry cross section and some technical devices of the rearing horse, we do not know whether the work ever progressed beyond the design stage. It can, however, be presumed that the solution proposed by Leonardo failed to completely convince Ludovico il Moro. In this regard, the latter wrote a symptomatic letter to Lorenzo de’ Medici, in which he asked for two skilled bronze-casters to be sent from Florence to Milan because he deemed Leonardo insufficiently skilled to complete such a project.
The second project: a 7-metre tall walking horse
Leonardo’s project changed course dramatically between 1489 and 1490. The statue’s dimensions almost tripled and the idea of a rearing horse was abandoned in favour of a horse in a walking posture.
Leonardo extensively researched the ideal proportions of the horse. He studied real horses in the Sforza stables as well as ancient and contemporary equestrian monuments in several Italian cities, such as Marco Aurelio in Rome, Regisole in Pavia, Bartolomeo Colleoni in Venice and the Gattamelata in Padua.