Well obviously a צפרדע is a frog. Or is it?
Ibn Ezra brings two explanations (Exodus 7: 27) - the first is that it is a type of fish found in Egypt that is called in Arabic "Altmesach" - which the commentator at the bottom of the "Torat Haim" torah translates as Crocodiles. This is interesting for two reasons - first of all, I have to say a plague of Crocodiles does sound somewhat more threatening than a plague of frogs, and secondly it seem Ibn Ezra thinks Crocs are fish (Which is fair enough considering the period he lived in).
The second explanation Ibn Ezra gives is that a Tzfardeah is " Those that are found in most rivers and make a noise - and this is the explanation I favor and is well known." in other words..frogs! I have no idea why he felt a need to describe frogs, and didn't just translate the word.