All Pokemon can Dynamax. What you might be thinking of is Gigantamax, which is the same thing but with a new form and unique move. Other than Zacian, Zamazenta, and Eternatus, all Pokemon can be dynamaxed/get supersized in raid battles, gyms battles, the Pokemon league, and the Battle Tower. Gigantamax Pokemon are special versions of Pokemon that can be gotten from gigantamax raids and from feeding certain Pokemon Max Soup.