When: 1917 Why: No.3A Autographic Kodak Special was the first coupled rangefinder camera ever
While rangefinders (or "telemeters") were introduced in the beginning of 1900s, they were used as optional accessories and were not linked to the taking lens (non-coupled). Their measured values had to be transmitted to the focusing scale manually.
Coupled rangefinder was "automatic" if I can say so. "Kodak Range Finder" (you can see it as a rectangular box under the lens) had three vertical mirrors in place (camera had to be held horizontally), and image in the middle one was shifting up or down when focusing was changed.
The whole new era started with this camera introduction. Rangefinders became an industry standard until SLR cameras didn't push rangefinders out.