The first AR-15 hanguards were triangular plastic, had aluminum heat shields inside, and were both right and left. They were originally green and then the color was changed to black. The first rifles to introduce the round, symetrical handguards were the XM177-series submachine guns (first fielded in late 1967 or early 1968). These handguards were black plastic with aluminum heat shields inside.
The symetrical, round handguards for the standard rifle were introduced by the M16A2 rifle. The M16A2 was fielded by the USMC in 1982. The handguards with the Mil-Std-1913 Picatinny rail have only made their appearance within the last five years.
Note: the cheaper plastic, non-military specification handguards may be found without the aluminum heat shields.