The most ancient records and art, Fremen or otherwise, fail to depict the larger species of cacti found on Arrakis. As near as I can conclude, these came from some ancient and abandoned botanical lab of the Old Imperium. Left by a dying civilization, these plants found their way out onto Arrakis, adapted to and subtly changed the environment. Life is tenacious and rude — it finds a way in, without invitation or hospitality.
Pardot Kynes, himself, modified and introduced plant species on Arrakis. I am trying, with some difficulty, to parse out which species came from him and which came from those lost labs. Those ancient structures are not all accounted for. What else grew down there? What else escaped?