A case may be made that they were the same historians & writers (and here I allude to people as Josephus, Philo, Tacitus...) to invent fictional ''historical'' men in order to better explain some caotic movements and cults of otherwise apparently unknown origin.Philo invented the Essenes as an ideal society and Josephus was much influenced by him,
Maryhelena has already mentioned Elior's thesis about who invented the essenes.
But there is another candidate about who invented just him: ''Judas the Galilean''.
It has been argued that both Judas and the Fourth Philosophy were not historical realities, but merely inventions of Josephus; see J.S. McLaren, ‘Constructing Judean History in the Diaspora: Josephus’s Accounts of Judas’s, in J. Barclay (ed.), Negotiating Diaspora: Jewish Strategies in the Roman Empire (London and New York: T&T Clark, 2004), pp. 90–108.
This scholar argues:
(p. 108, my bold)This study shows that we can no longer assume that this Judas presented by Josephus is an historical figure who engaged in some activity in 6 CE. It is not simply a case of claiming that Josephus may have exaggerated the account of Judas' career and its impact by adjusting a few details here and there. Rather, Josephus's apologetic has constructed Judas, making him a vital part of the explanation of what happened in Judaea in 66-70 CE. Who he was, what he did and what he advocated, if anything at all, need to be established afresh, outside the framework provided in War and Antiquities.
Therefore maybe Couchoud was right about Tacitus: he invented Jesus to explain (possibly in a rationalistic way) the origin of the name of the Chrestiani (we would call it euhemerism).
The logic of Tacitus could be the following:
1) I have not the faintest idea who they were these Jews killed by Nero after the Great Fire.
2) maybe they were the same Christians who preach in Rome today.
3) these Christians mourn the death of their crucified 'Christ'.
4) if this crucified Christ is nevertheless revered by Christians, then it must have been that cruel and inept governor named Pilate to crucify him by mistake (since he killed so many Jews indiscriminately, according to Josephus).
5) therefore: the ''more simple explanation'' for the origin of the name Chrestiani (auctor nominis eius...) is that they derived by a certain 'Christ' crucified by Pilate.
That euhemerist idea introduced by Tacitus took root among the Pagans largely as anti-Christian mockery, and later Marcion was inspired by that idea, and wrote a story...