The Ghostlight is the "antagonist" in Mater and the Ghostlight.
The Ghostlight is mentioned twice by Mater in this movie. The first time is when Mater and Lightning McQueen were coming back from tractor-tipping. The second time is when a helicopter searchlight pointed at McQueen, and Mater thought it was the Ghostlight.
Mater and the Ghostlight
The Ghostlight is a glowing orb of blue, translucent light.