Almost every Specter is a black flying creature with a "saboteur" ability (an ability that triggers when the creature gets through unblocked, or deals combat damage) that causes the opponent to discard. And almost every Specter is depicted as a brooding, hooded figure riding a flying mount, an undead creature whose frightening appearance and Specter's Wail causes the mind-draining discard effect.
Abyssal Specter in Ice Age was the earliest example of the flying mount convention for Specters. In Magic's early years, that spectral flavor link was not yet established, so there are a few counterexamples to this pattern. Tainted Specter from Mirage is a flyer with a discard ability, but isn't depicted as a mounted figure. Same with Entropic Specter from Exodus. Mirage's Dread Specter is probably the strangest oddball of the bunch—like honorary Specter Mistform Ultimus, it isn't mounted, doesn't fly, and doesn't have a discard ability!
The original Specter, Alpha's Hypnotic Specter, was reillustrated to bring it in line with the other recent Specters when it resurfaced in Ninth Edition. What was originally a ghostly knight is now firmly part of the Specter tradition, as a hooded figure riding a flying mount.