Sforno on Exodus 8:3/ספורנו על שמות ח׳:ג׳

1ויעלו את הצפרדעים, the ones produced by the sorcerers were unable to reproduce themselves, as were those that Aaron had produced. The sorcerers were never able to produce any creature that could move on its own. [It is important to realise that our author considers the צפרדע as a type of crocodile, the major danger to humans and fish inhabiting the Nile. Ed.]