About what fraction of William's army was cavalry?

About one-quarter of William’s army at Hastings was cavalry.

The Norman force had a mixed structure unusual for the largely infantry-based English army. Wikipedia describes William’s army as roughly half infantry, one-quarter cavalry, and one-quarter archers or crossbowmen. Exact troop totals remain uncertain, but historians generally agree about this broad division.

Cavalry played an important role in the battle’s later stages. Norman horsemen repeatedly attacked the English position and were involved in feigned retreats, in which troops appeared to flee and then turned on English soldiers who broke formation to pursue them. These tactics helped weaken the shield wall.

A common mistake is to assume that William’s army was mostly cavalry because mounted knights dominate popular images of the Norman Conquest. In reality, infantry and missile troops made up about three-quarters of the force, while cavalry formed approximately one-quarter.