I think that Scott Greene, in his otherwise excellent online resource for converted classic D&D monsters, the "Creature Catalog" (and now being printed as the Tome of Horrors) succumbed to one unfortunate recurring mistake. And that is: basically all of the "giant" animal-types (for example, the giant eel, frog, snapping turtle, and so forth) in his work have been given the "Beast" type.
There's no compelling reason for this. Creatures such as these in the Monster Manual are consistently given the "Animal" type. The huge dire animals are all Animals, not Beasts. Giant crocodiles, lizards, octopi, snakes, and squid are all Animals, not Beasts. If one looks at "Monsters by Type" index on MM p. 4, one sees that to be a Beast one needs some exceptional or fantastic monstrous qualities: such as the ankheg, bulette, griffon, owlbear, and so forth. Dinosaurs (from the age of reptiles) are Beasts, but, as clearly implied in the dire animals entry, merely prehistoric monsters (from the age of mammals) should be Animals. Creatures such as the giant eagle or owl are categorized as Magical Beasts only because of their exceptional intelligence and use of language.
Compared to Animals, Beasts get an increased Hit Die type (d8 to d10), along with darkvision and an added "good" saving throw category -- so the difference is not insignificant. I very much believe it would be more compatible with the core rules if these creature types were converted as "Animals", not "Beasts".