Larry Graham was recognised as the inventor of slapping on the electric bass, although some people argue that there were people before him and he just popularised it.
Before Larry Graham was slapping on the electric, there were people like Milt Hinton who were slapping the upright bass.
Although Flea didn't invent slapping, he did develop a style that was all his own. He attacked the strings with the ferocity and speed of a punk player, while still maintaining the groove of his jazz roots.