What was the first Pixar short film featuring characters from Cars?

Mater and the Ghostlight was the first Pixar short film featuring characters from Cars.

The short was released in 2006 as a companion to Cars and centers on Mater, the tow truck from Radiator Springs. After the town's residents play a prank on him, they warn him about the Ghostlight, a mysterious blue light said to haunt the area. The story is a compact supernatural comedy built around Mater's imagination and the town's folklore.

Unlike the later Mater's Tall Tales series, Mater and the Ghostlight is a standalone short tied directly to the first feature. It was included with home-video editions of Cars and also appeared in some related releases. The short helped extend the world beyond the feature without changing its main storyline.

The title is sometimes confused with the later shorts Rescue Squad Mater, Tokyo Mater, and other Mater's Tall Tales entries. Those came afterward, so they are not the first Cars short.