This passage above suggests that it is a ‘fact’ that one of Pyrrhus’ advisors made such a comparison. The story is known from Cassius Dio (9.40.27):
The same man, when, upon his retreat, he beheld the army of Laevinus much larger than it had been before, declared that the Roman legions when cut to pieces grew whole again, hydra-fashion. This did not, however, cause him to lose courage, but he in turn arrayed his forces, though he did not join battle.
It is said, too, that Cineas, while he was on this mission, made it his earnest business at the same time to observe the life and manners of the Romans, and to understand the excellences of their form of government; he also conversed with their best men, and had many things to tell Pyrrhus, among which was the declaration that the senate impressed him as a council of many kings, and that, as for the people, he was afraid it might prove to be a Lernaean hydra for them to fight against, since the consul already had twice as many soldiers collected as those who faced their enemies before, and there were many times as many Romans still who were capable of bearing arms.
Appian pulls these two traditions together:
The Senate made answer to Cineas as Appius advised. They decreed the levying of two new legions for Laevinus, and made proclamation that whoever would volunteer in place of those who had been lost should put their names on the army roll. Cineas, who was still present and saw the multitude hastening to be enrolled, is reported to have said to Pyrrhus on his return: “We are waging war against a hydra.” Others say that not Cineas, but even Pyrrhus himself said this when he saw the new Roman army larger than the former one; for the other consul, Coruncanius, came from Etruria and joined his forces with those of Laevinus.
Appian makes clear that bon mot was not a fixed point in the received tradition. He knew it from at least two different sources with different variations. We can’t be sure if Appian’s sources were riffing on Silenus’ motif or faithfully recording an actual piece of rhetoric from the time or if the metaphor is just so pervasive that it provides a nice plausible exclamation in any history.
Heck. There are dozens upon dozens of popular histories to day that still use the metaphor. The loose use of the metaphor is found in many earlier Greek works including Plato’s Republic, p426E.
All that said, this Florus passage (going back to a lost bit of Livy?) might be the best evidence that some lost historian made something of the Pyrrhus = Hercules, Rome = Hydra symbolism on a more meaningful level that a simple metaphor.
For Pyrrhus said, “I plainly see that I am sprung of the seed of Hercules, when I see all these heads of foes cut off springing up again from their blood as they sprang from the Lernaean hydra.”
Perhaps tellingly for the attribution to Pyrrhus, Plutarch uses it when discussing the actions of Alexander.
The use of metaphor in relationship to Pyrrhus is not irrelevant to a discussion of Silenus, but I’d hesitate to move it from a conversation about the historiographical tradition and into one about propaganda.
Note also how the hydra in Pyrrhus tradition is not a negative characterization of Rome, not emphasizing her monstrosity or destructive capacity, but instead resilience and depth of martial resources, especially her manpower base. It’s a complement.