Straight from the Hertz website:
Toyota Prius Hybrid
Reserve this Exact Model, ICAH
Are you using a coupon that's only good up to full size? I highly recommend finding one that will work on the green collection (I just got a $35 off weekly green collection coupon playing the Hertz game on their website) because I reserved an F in SFO and tried to get into a hybrid at pickup and there were NONE available. I saw lots of Camry hybrids being driven out as I left the rental facility. They're popular!