Originally Posted by HTupolev

I'm not sure that that can be well-characterized as an "origin." The Cambio Corsa was a rare example of a derailleur that required a horizontal dropout, but it's not where horizontal dropouts started. For early bicycles, horizontal fixtures for the rear wheel were the norm, since they're the most minimal way to provide the ability to tension a chain. Track ends were initially more common, but by the 1930s, "modern" horizontal dropouts were making their mark; this meant that they were being used with all kinds of drivetrains, including derailleurs. For instance, here's a 1934 advertisement for Cyclo, showing their Witmy derailleur being used on a bicycle with horizontal dropouts.