Wikipedia
The were a group of four troublesome demons who created obstacles and difficulties in Hindu mythology, but who were easily propitiated. One theory of the origin of Ganesha is that he gradually came to prominence in connection with the .
In Puranic literature of a much later period the group of four Vināyakas was merged into one definite god named Vināyaka whom Rudra appointed as the "Leader of the Ganas" (Ganapati). This Vināyaka-Ganapati is associated with another god called Dantin, "the one with the tusk," who is said to possess a twisted trunk and who holds a corn-sheaf, a sugar cane, and a club. This description of Dantin is so characteristic of the Puranic Ganapati that Heras says "we cannot resist to accept his full identification with this Vedic Dantin." The name Vināyaka is a common name for Ganesha both in the and in Buddhist Tantras.
In the Smrti of Yājñavalkya, written in the 6th century, Vināyaka is definitely mentioned as a demon who had been exalted to the rank of a deva. He is clearly described as elephant-headed by the 8th century.