Morannon Orcs are one of the largest type of Orcs, originating from Mordor, but were not necessarily all bred there at the Morannon, the Black Gate of Mordor. They were also called black orcs by some men and elves. The term "Morannon Orc" did not come from Tolkien's works, but from the media-canon of the LOTR New-Line films.
History
Morannon Orcs were the shock troops of Mordor and appeared in most of the main battles such as the Battle of Osgiliath, the Siege of Gondor, Battle of Cair Andros and the Battle of the Black Gate. In The Lord of the Rings: The Return of the King (2003) they marched out of Minas Morgul when they left Mordor to lay siege to Minas Tirith.
Physical Attributes
They wore black chainmail and iron. They were among the largest size Orcs in Sauron's army and also the most common. Though lower in rank than the rarer Black Uruks in Sauron's army, they were very tough, and were probably the most numerous type of orc in Sauron's army at the Pelennor Fields, which numbered some 200,000 including the Mannish and troll-kind allies, although other types such as the Snagae and ambiguous breeds such as Morgul orcs, combined, were more in number. Morannon Orcs were also trained for more technical military tasks than other Orcs, tasks such as building siege weapons and boats.
Morannon Orcs as portrayed in the BFME game series, as "Black Orcs"