Exerpt From Wikipedia:

Both Shakespeare and Samuel Ireland identify Herne as a real historical individual, the latter saying that he died an unholy death of the type that might have given rise to tales of hauntings by his unquiet spirit. The fact that Herne is apparently a purely local figure supports this theory. One possibility is that Herne is supposed to be the ghost of Richard Horne, a yeoman during the reign of Henry VIII who was caught poaching in the wood. This suggestion was first made by James Halliwell-Phillipps, who identified a document listing Horne as a "hunter" who had confessed to poaching.^[17] The earliest edition of The Merry Wives of Windsor spells the name "Horne".^[18]

For me this is the most believable and likely origin of the name "Herne the Hunter", although I suspect as do many today that the legend of Herne has grown and fused so closely to Cernunnos that the 2 are indeed one and the same.