As mentioned, these things happen for various reasons.

Also agreed that chances are pretty high you'll be initially offered a Camaro if renting from Avis. Seems to be their go-to car in that category.

At the same time, I found the lot at Avis LAX to be helpful on multiple occasions where I wanted / needed a particular vehicle. They have a habit of not assigning cars for Preferred customers, so you will have a chance to talk to an agent at the counter anyway. They have lots of inventory coming through, and if you're nice enough and offer to wait, I'd say you have an excellent chance of driving off with a Mustang. If all fails, call a couple of Avis stations along your route and ask them if they have a Mustang they're willing to trade you for your Camaro.