I know that the 4th plague in Egypt, Arov, was a mixture of wild beasts. At least, that is what Rashi and all the other commentators I looked at explain. However, ALL non-Jewish bible translations and several Jewish ones including Soncino’s "The Pentateuch and Haftora" (with translation of late chief rabbi of England Hertz) translate this plague as "swarm of flies" Da’at Mikra states that "most commentators explain it as wild beasts, but few of the "kadmonim" and many achronim explain that is was a swarm of flies" What is the source for this? Who are these kadmonim and achronim? And why does EVERY Jew nowadays knows that it means wild beasts and not flies?
In Tehilim 78, 45, it says "will send Arov and it will eat them”, inferring that the 4th plague was beasts, as Rashi opines in the name of the midrash. On the other hand, the Targum Shiv’im and Targum Akilas (student of R. Akiva), two of the oldest authoritative rabbinic Aramaic translations, both explain that Arov is flies or mosquitoes, as is the opinion of the ancient Tanna, R. Nechemya, cited in the same (!) Midrash Tanchuma, VaEra 14, which cites the alternative opinion, quoted by Rashi. Among the rishonim, the Abarbanel cites both opinions, the B’chor Shor (student of Rabbenu Tam) agrees with R. Nechemya, as does Shadal and other recent commentators, for thus it’s understood why the animals couldn’t find shelter behind closed doors. Also, it’s more applicable to what’s written twice about Arov, that the ground (not the land!) was full of them. Accordingly, explains Shadal, the aforementioned proof from Tehilim is speaking rhetorically in exaggeration, as in the continuation of the verse there: “… frogs which destroyed (!) them”. “Flies” would also fit well with Tehilim 105, 31, which pairs Arov together with lice which “filled all their borders”. Chazal tell us that what we learn as youngsters remains forever, and we all grew up on Rashi, so that explains why that’s the more popular of these two rabbinic explanations.