From the rule book 1.04.
1.09.34 MASTER OF HORSE: Whenever the Dictator commands
a Force, he is accompanied by his Master of Horse. The Dictator’s
Military rating for any combat is the sum of his Military rating and
that of his Master of Horse. The Master of Horse must accompany
the Dictator to War and cannot act independently of the Dictator,
unless the Dictator dies in combat or if the Dictator Revolts (1.11.3),
he remains loyal to the Dictator. If victorious in a War, the Dictator
receives the usual increase in his Influence and Popularity (1.10.4),
while the Master of Horse gets nothing. If the Dictator is defeated,
the Master of Horse is killed along with him. The Master of Horse can
also be killed by a Mortality Chit draw during a battle result (1.10.7).
The Master of Horse also adds to the Commander’s Military rating in
a Naval Battle (reflecting the greater efficiency of a unified command),
but cannot be used to nullify a Disaster or Standoff result in any battle
So the master of horses role sucks. You get nothing if you win, and risk the chance of death the same thing as a normal military commander, worst even.