Does Frontier fly into ONT?
 

I see CR7's landing with Frontier logo but don't see the airport on their website.

Can't be F9. They made a big deal this spring of being 100% AirBus, giving up the last of the old 37's, and AFAIK, no regionals in the fleet. Look for the animal painted on the tail.

Frontier does fly CR7's under the name Frontier JetExpress. Frontier JetExpress flies to GEG, BOI, BIL, SJC, TUS, ELP, ABQ, AUS, LIT, OKC, TUL, OMA, and starting August 31st to DAY and FAT. You are right that all of Frontier mainline is Airbus.

Horizon operates the Frontier JetExpress fleet. I have heard that Horizon has on occasion swapped RJ equipment between Frontier and Alaska. Maybe a Frontier aircraft is filling in on ONT-PDX?

Frontier use to fly to Ontario, but pulled out in October of last year and used that equipment to Little Rock. The fare battle between Ted on the route made it unprofitable and F9 did not want to continue matching prices which did not come close to covering the cost of the flight.

Occassionally a QXE CR7 painted with Frontier JetExpress will be used for Horizon flights on the coast, as well as a flight to Portland for MX from Denver.