The blaze that started last Friday has destroyed 12 structures and injured one firefighter.
On Tuesday, Orange County Professional Firefighters said on Instagram that a lost Queensland heeler was discovered while crews were battling the blaze.
"They tended to the dog, gave him water, a bite of their sandwich and carried down the fire line to the animal control representative," the firefighters union wrote.
A firefighter could be seen holding the dog while descending from the mountain.
The Riverside County Department of Animal Services told KTLA the dog was found to have been reported missing and was reunited with its owner.
Some evacuation orders were lifted Tuesday in Riverside County but still remain in San Bernardino County as the fire continues to burn.
The wildfire forced thousands of people from their homes after it was sparked by a malfunctioning diesel vehicle, fire officials said Monday.
"Specifically, a diesel-fueled vehicle emitting burning carbon from the exhaust system," the agency said. "The determination is reinforced by multiple independent witnesses, as well as supporting physical evidence."
The wildfire generated a smoke plume that was visible for miles around and contributed to poor air quality.
Firefighters battling the blaze have faced "dangerously hot conditions” that are sticking around. The heat on Wednesday will scorch the Southwest and southern Plains, with temperatures at or above 100 degrees.
An elevated risk for fire danger once again will be present for the Southwest into the Great Basin and Central Rockies.
Dry thunderstorms will also be a threat for potential wildfires over northern California up into the Northwest.
Fox News' Janice Dean and the Associated Press contributed to this report.