The Romans of the Republic were surprisingly superstitious, and Sertorius was considered particularly favored by the Gods because of his white fawn. Which he tended to take advantage of ... [Plutarch, Sertorius, 11-12.1]:
Most of the people joined him of their own accord, owing chiefly to his mildness and efficiency; but sometimes he also betook himself to cunning devices of his own for deceiving and charming them. The chief one of these, certainly, was the device of the doe, which was as follows.
Spanus, a plebeian who lived in the country, came upon a doe which had newly yeaned and was trying to escape the hunters. The mother he could not overtake, but the fawn — and he was struck with its unusual colour, for it was entirely white — he pursued and caught. And since, as it chanced, Sertorius had taken up his quarters in that region, and gladly received everything in the way of game or produce that was brought him as a gift, and made kindly returns to those who did him such favours, Spanus brought the fawn and gave it to him.
Sertorius accepted it, and at the moment felt only the ordinary pleasure in a gift; but in time, after he had made the animal so tame and gentle that it obeyed his call, accompanied him on his walks, and did not mind the crowds and all the uproar of camp life, he gradually tried to give the doe a religious importance by declaring that she was a gift of Diana, and solemnly alleged that she revealed many hidden things to him, knowing that the Barbarians were naturally an easy prey to superstition.
He also added such devices as these. Whenever he had secret intelligence that the enemy had made an incursion into the territory which he commanded, or were trying to bring a city to revolt from him, he would pretend that the doe had conversed with him in his dreams, bidding him hold his forces in readiness. Again, when he got tidings of some victory won by his generals, he would hide the messenger, and bring forth the doe wearing garlands for the receipt of glad tidings, exhorting his men to be of good cheer and to sacrifice to the gods, assured that they were to learn of some good fortune.
12 By these devices he made the people tractable, and so found them more serviceable for all his plans; they believed that they were led, not by the mortal wisdom of a foreigner, but by a god. At the same time events also brought witness to this belief by reason of the extraordinary growth of the power of Sertorius.
These white bucks are Judas deer [source]; white deer are rare today, and were even more so in antiquity when whiteness was greatly admired.
Sertorius led an interesting life. He was one of the few to survive Arausio (105 BC) Marius' spy during the German Wars. After Marius' death he defied Sulla, fighting on in Spain and North Africa. When he needed help, he made a deal with the Cilician pirates [Plutarch, Sertorius, 8.3]:
Sertorius sailed back again to Spain. From this shore too he was repulsed, but after being joined by some Cilician piratical vessels he attacked the island of Pityussa, overpowered the guard which Annius had set there, and effected a landing.
They worked again against Metellus and Pompey the Great [Plutarch, Sertorius, 21.5]:
and so once more he advanced upon the enemy with large reinforcements and began to cut off their land supplies by means of ambuscades, flank movements, and swift marches in every direction, and their maritime supplies by besetting the coast with piratical craft; so that the Roman generals were compelled to separate, Metellus retiring into Gaul, and Pompey spending the winter among the Vaccaei.
Sertorius deserves a full biography, but for now the best read about him is Adrian Goldsworthy's chapter on him in:
In the Name of Rome: The Men Who Won the Roman Empire (Phoenix Press)- Amazon.co.uk
In the Name of Rome: The Men Who Won the Roman Empire (Phoenix Press) - Amazon.com