Augustus was not the first emperor. There had been previous dictators, the first which was Horatio Cockles who at the bridge over the Tiber, stopped the Tarquin kings from being reinstalled.
Julius Caesar, Crassus, Pompey, Sulla, Gaius Marius had all been dictator (Imperator) at some time.
Augustus never referred to himself, nor allowed anyone else to refer to him,as ‘Emperor’. He called himself ‘Princeps’.
All imperators, not “emperors.” Octavian was made Augustus Princeps, which was an unprecedented title created for him.
Dictator was a constitutional office under the Republic. It granted absolute and unquestionable power for six months, and required the Senate to pass a decree authorizing the consuls to appoint someone. The law establishing the office was passed in 501 BC, five years after the Republic was founded.
Neither Crassus, Pompey nor Marius ever held this office, although Marius briefly held unconstitutional absolute power, acting functionally as a dictator in today's misuse of the word. Also Pompey and Crassus were triumvirs with Caesar, and thus held a joint unconstitutional dictatorship, again in the modern sense. The Roman dictatorship was by definition held by only one man, so the triumvirs weren't really dictators.
Sulla and Caesar both became dictator using violently unconstitional means and also held office long beyond the constitutionally prescribed six months, Caesar eventually appointing himself dictator for life.
The term Imperator, which later became Emperor, was an acclamation made of a victorious general on the field of battle by his troops. It is roughly translated "commander," and implied their acknowledgement of his earning of this title.
Hundreds of Romans were acclaimed Impertor before Augustus came along. Being acclaimed Imperator on the field of battle was a pre-requisite for a general to receive a triumph.
As far as I know, Horatius was never a dictator, or even a politician. One story is that he drowned trying to get back across the river.