Gillray's "The Farmer's Friend" (1792) is often cited as a groundbreaking political cartoon. It satirized King George III, depicting him as a butcher, and used strong visual symbolism to comment on the political situation of the time.
While Gillray's work was significant, it's important to note that political satire existed before him in forms like woodcuts and broadsheets. However, Gillray's style and influence on the development of the political cartoon make him a key figure in its history.