Plutarch Galba 23

[1] Thus was Vitellius proclaimed emperor in Germany; and when Galba learned of the revolution there he no longer deferred his act of adoption. Knowing that some of his friends favoured the selection of Dolabella, and most of them that of Otho, neither of whom was approved by himself, he suddenly, and without any previous notice of his intention, sent for Piso (whose parents, Crassus and Scribonia, had been put to death by Nero),

[2] a young man in whose predisposition to every virtue the traits of gravity and decorum were most conspicuous; then he went down to the camp to declare him Caesar and heir to the throne. And yet as soon as he set out, great signs from heaven accompanied him on his way, and after he had begun to pronounce and read his address to the soldiers, there were many peals of thunder and flashes of lightning, and much darkness and rain pervaded both the camp and the city, so that it was plain that the act of adoption was inauspicious and was not favoured or approved by the heavenly powers.

[3] The soldiers also were secretly disloyal and sullen, since not even then was their largess given to them.
  As for Piso, those who were present at the scene and observed his voice and countenance were amazed to see him receive so great a favour without great emotion, though not without appreciation; whereas in the outward aspect of Otho there were many clear signs of the bitterness and anger with which he took the disappointment of his hopes. He had been the first to be thought worthy of the prize, and had come very near attaining it, and his not attaining it was regarded by him as a sign of ill-will and hatred on Galba’s part towards him.

[4] Wherefore he was not without apprehension for the future, and fearing Piso, blaming Galba, and angry with Vinius, he went away full of various passions. For the soothsayers and Chaldaeans who were always about him would not suffer him to abandon his hopes or give up altogether, particularly Ptolemaeus, who dwelt much upon his frequent prediction that Nero would not kill Otho, but would die first himself, and that Otho would survive him and be emperor of the Romans (for now that he could point to the first part of the prediction as true, he thought that Otho should not despair of the second part). Above all, Otho was encouraged by those who secretly shared his resentment and chagrin on the ground that he had been thanklessly treated. Moreover, most of the adherents of Tigellinus and Nymphidius, men who had once been in high honour, but were now cast aside and of no account, treacherously went over to Otho, shared his resentment, and spurred him on to action.