More than 500 firefighters are battling the flames, which are burning near Interstate 15, a major route connecting Las Vegas to Southern California.
The blaze, which is about 5 percent contained, broke out about 2 p.m. and had spread across about 1,500 acres by Saturday night.
Its cause is unknown.
An evacuation center has been established at Eisenhower High School at 1321 N. Lilac in Rialto.
Large animals may be taken to the Glen Helen Rodeo grounds and small animals may be taken to the Devore Animal Shelter.