Cnidarian life cycle, a case of ontogenetic polymorphism?
Sure it would.  I would usually use holometabolous insects as an example of this (egg, larva, pupa, adult), but Cnidarians certainly fulfill the basic requirements encapsulated by the phrase "ontogenic polymorphism." It appears that the pressures of finding/establishing food sources and escaping predation in competitive environments has driven both Cnidaria and Insecta toward this evolutionary strategy, though that is a field of active study still (at least in insects).