In the Gathering Storm expansion, the Hetairoi requires 10 Horses to train.
Unlike the Horseman it replaces, the Hetairoi is classified as a heavy cavalry unit, which means they can be upgraded into Knights. This means any Hetairoi you build can stay relevant for a long time, from the early game through the mid-game, into the Industrial Era. For Alexander, there is only one way to victory: fight with your neighbours to generate Great General points, conquer their cities to trigger Hellenistic Fusion's bonus, and use these Eureka moments to speed through the tech tree, and keep your units up-to-date and your army powerful.
The powerful hetairoi (known simply as “the Companions”) were an elite cavalry unit who served as the hammer in Macedon’s famed hammer-and-anvil tactics, popularized under Philip II and used to great effect by his son, Alexander the Great. While the Macedonian phalanx pinned a rival army in place with their extra-long spears (known as sarissas), the hetairoi would organize into a wedge and charge directly into the enemy flanks or rear.
An abrupt attack by the shock cavalry nearly impossible to avoid or deter, and was often enough to break the will of less seasoned troops. As they fled for their lives, they were easy pickings for the hetairoi who could ride them down and dispatch them in their leisure.
Most of the Companions came from the ranks of Macedon’s noble families, though the king could technically admit anyone to their ranks. They were armed with short spears and swords.