The pork barrel appears to have been first used by none other than Pericles.
Pericles' greatest political rival was a man called Cimon. Cimon was an enormously wealthy arch-conservative who was convinced this democracy thing was a big mistake, and that governance of the city should be left in the hands of the better aristocrats. That wouldn't have been a challenge to Pericles, except that Cimon was extremely generous to his fellow citizens, funded major public works from his own pocket, and was even responsible for building the Academy, which later became the school of Plato. To top it off, Cimon's dad was Miltiades, the man who led the Athenians at the battle of Marathon, and Cimon himself was a war hero. Cimon was very, very popular with the people.
Pericles desperately had to do something to counter Cimon, as a me-too gesture.
This from The Athenian Constitution, by Aristotle, from the excellent Penguin edition:
Pericles was the first man to provide payment for jury service, as a political measure to counter the generosity of Cimon. Cimon was as rich as a tyrant: he performed the public liturgies lavishly; and he maintained many of his fellow-demesmen, for any man of Laciadae who wished could go to him each day and obtain his basic needs, and all his land was unfenced, so that anyone who wished could enjoy the fruit.Pericles' property was insufficient for this kind of service. He was therefore advised by Damonides of Oe (who seems to have been the originator of most of Pericles' measures, and for that reason was subsequently ostracized) that since he was less well supplied with private property he should give the people their own property; and so he devised payment for jurors.
It looks like Damonides of Oe gets the gong for the world's first known spin doctor.
Like most intellectuals of the day, Aristotle was no egalitarian democrat. He then had this to say:
Some people allege that it was as a result of this that the courts deteriorated, since it was always the ordinary people rather than the better sort who were eager to be picked for jury service.