Darius certainly wasn't the first horizontally scrolling shooter, or probably even the first that comes to mind for most people. It wasn't even the first shooter from developer/publisher Taito. But it was one of the most innovative, and that's mostly because of the arcade hardware involved. Much like Taito's own The Ninja Warriors
one year later, Darius was housed in a sprawling setup: Three either 19 or 15 inch monitors (later the 15 inch version would be the only one still in production), a "body-sonic" seat with speakers beneath it to make the player really feel the music, and even a headphone jack (not a common feature on arcade games at this point). Even with modern arcade hardware, it's very rare for a cabinet to be so focused on presenting both a superior audio and visual experience.