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The Master of the Horse was (and in some cases, still is) a position of varying importance in several European nations.

The original Master of the Horse (Latin Magister Equitum) in the Roman Republic was an office appointed and dismissed by the Roman Dictator, as it expired with the Dictator's own office, typically a term of six months in the early and mid-republic. The Magister Equitum served as the Dictator's main lieutenant. The nomination of the Magister Equitum was left to the choice of the Dictator, unless a senatus consultum specified, as was sometimes the case, the name of the person who was to be appointed. The Dictator could not be without a Magister Equitum to assist him, and, consequently, if the first Magister Equitum either died or was dismissed during the Dictator's term, another had to be nominated in his stead.

The Magister Equitum was granted a form of imperium, but at the same level as a praetor, and thus was subject to the imperium of the Dictator and was not superior to that of a Consul. In the Dictator's absence, the Magister Equitum became his representative, and exercised the same powers as the Dictator. It was usually but not always necessary for the man nominated as Magister Equitum to have already held the office of Praetor. Accordingly, the Magister Equitum had the insignia of a praetor: the toga praetexta and an escort of six lictors. The most famous Master of the Horse is Mark Antony, who served during Julius Caesar's first dictatorship.

After the constitutional reforms of Augustus, the office of Dictator fell into disuse, along with that of the Magister Equitum. The title magister equitum was revived in the late Empire, when Constantine I established it as one of the supreme military ranks, alongside the Magister Peditum ("Master of the Foot"). Eventually, the two offices would be amalgamated into that of the Magister Militum ("Master of the Soldiers").

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Wikipedia