Stegosaurus sulcatus—"furrowed roof lizard"—was described by Marsh in 1887 based on a partial skeleton, but it may be a synonym of Stegosaurus armatus. Stegosaurus ungulatus—"hoofed roof lizard"—was named by Marsh in 1879 for a few vertebrae and armor plates from Como Bluff, Wyoming. It may represent a juvenile form of Stegosaurus armatus, but this didn't deter Escaso et al. from assigning a Portuguese specimen here well over a century later, which makes it the first and so far only confirmed Stegosaurus specimen known from outside of North America, regardless of which species it represents. Stegosaurus affinis—"Related roof lizard"—is only known from a pubis found in "Quarry 13" at Como Bluff which was never illustrated and only poorly described by O.C. Marsh in 1881. It was never assigned a specimen number either, and its remains appear to be lost. It may be synonymous with Stegosaurus armatus. Stegosaurus seeleyanus—"Seeley's roof lizard"—was originally named Hypsirophus seeleyanus, a second species of Hypsirophus—which isn't a Scooby Doo typo of Hypsilophus—by E.D. Cope in 1879. Although never illustrated or actually described, E.D. Cope renamed its Stegosaurus seeleyanus later the same year, though its remains probably belong to Stegosaurus armatus. Stegosaurus laticeps—"Broad-headed roof lizard"—was originally described by Marsh in 1881 as Diracodon (neck-point tooth) based on some jawbone fragments. Following a 70s/80s trend of combining dinosaur names—Edmontonia (Chassternbergia) rugosidens, Panoplosaurus (Edmontonia) longiceps, and so on—Bob Bakker semi-resurrected Diracodon laticeps as Stegosaurus (Diracodon) laticeps in 1986, though most paleontologists consider the material to be non-diagnostic and likely synonymous with Stegosaurus stenops. Stegosaurus duplex—"two plexus roof lizard" (YPM 1858)— started the "two brained" theory. Collected in 1879 by Edward Ashley at Como Bluff but not named until 1887, a greatly enlarged canal in its sacrum (YPM 1857) was interpreted by O.C Marsh as a "posterior brain case". It's probably the same as Stegosaurus armatus, though Marsh initially thought it belonged to Stegosaurus ungulatus. Stegosaurus madagascariensis—"Madagascan roof lizard"—was described by Piveteau in 1926 based solely on teeth that have variously been identified as the property of a theropod (probably Majungasaurus), a hadrosaur and a crocodylian. Stegosaurus marshi—"Marsh's roof lizard"—was named and described briefly by Lucas in 1901 based on a mangled skeleton found by Nelson Horatio Darton at Buffalo Gap Station in the Lakota Formation of Custer County, South Dakota, in 1898. It was renamed Hoplitosaurus by Lucas in 1902 and fully described by Gilmore in 1914, but continues to court controversy. Stegosaurus priscus—"Ancient roof lizard"—was described by Nopcsa in 1911. It was reassigned to Lexovisaurus by Galton in 2004, but is now the type species of Loricatosaurus. Stegosaurus longispinus (Gilmore, 1914) was controversially renamed Natronasaurus longispinus by Ulansky in 2014... which was controversially renamed Alcovasaurus by Galton and Carpenter in 2016. Funnily enough, during a cull of stegosaurids by Maidment in 2008, she put down her scythe long enough to raise two Stegosaurus species of her own, though few experts have accepted them:
Stegosaurus mjosi, based on a critter previously known as Hesperosaurus mjosi and Stegosaurus homheni, based on a critter long known as Wuerhosaurus homheni.