Scimitars in HaradEdit
The Haradrim are the civilization most commonly described as using scimitars, with their cavalry bringing scimitars onto the Pellenor. In the books, they seemed to have made scimitars out of steel, but in the movies the only metal they have is gold plates worn as trophies. Through a movie standpoint, the Haradrim would have likely made these scimitars by combining a stone, bone, or obsidian blade with a bamboo, wood, stone, or bone handle. These scimitars can typically be 3 foot 6 inch in length, with the blade being blunt for the first 10 inches to avoid self-laceration (exposing 4 inches of blunt blade and 30 inches of sharp blade)
The Scimitar in Mordor and RhûnEdit
The Easterlings have developed a metal version of the Haradrim's organic scimitar: a steel-bladed brass handle that can cause a trainwreck among sword-made damages. It was standard issue to every soldier in the Host of Rhûn. This included any siege machine crew. It can be speculated that the Easterlings were trained how to use it by the Haradrim during the Second Age, using the thrust and the cut to slice through enemy lines. This made the Easterlings a much more significant threat to the Northmen, especially during the War of the Ring, where both the rural soldiers of Rohan and the urban soldiers of Dale were under the threat of Easterling armies, 6,000 Rohirrim facing 7,000 Easterlings, while the Bardings and their Dwarven allies faced an Easterling-only army of 200,000.
As for Mordor, the Orcs would have likely made several straight blades and curved blades including the design depicted above and the designs described in the etymology headline. Granted, these scimitars would have been of the lowest quality: unable to damage leather armor, needing frequently to be repaired even after an orc victory (orc victories being almost nonexistent), and unable to sever bone.
Tolkien uses the term "scimitar" to describe any curved blade existing in Middle-earth. However, there are other kinds of curved blades, including forwards-curving ones like the Kopis and the Falcutta. Some backwards-curving blades do not count as scimitars in swordsmanship, but in the case of Middle-earth as Tolkien describes it, any' curved blade is a scimitar. The scimitar in the picture is actually a falchion; a scimitar is a backwards-curving blade on a J-shaped handle with a cross-guard that curved up on the sharp side and down on the dull side. The blade curve was asymmetrical to the handle curve.