The official plural for the Toyota Prius is Prii.
Toyota had an official press release for this back in 2011. Other contenders for the name?
Prius
Priuses
Prium
Prien
It looks like Toyota wanted to stick with a single word, but “Prius vehicles” or “Prius models” would be another option if you were influenced by, “some kind of fruit company” that uses very similar nomenclature when mentioning their products.
The grammatically correct answer by the way is Priuses.