GMC Caballero
Also known as the “Sprint,” the Caballero was the first “car” to ever have a GMC badge on it. The car shared its platform with the El Camino and most of the styling queues. The engine was also shared between the two cars. The problem was that the performance of the El Camino was lackluster itself and there wasn’t a reason to offer a GMC version. The Caballero was one of the worst-selling GMC models of all time (via Silodrome).
There’s quite a collectors market for these cars, although it pales in comparison to what the El Camino goes for these days. The Caballero didn’t offer much in the way of uniqueness to differentiate it from the El Camino. If GM offered more performance and unique styling, the Caballero might have done better.