New Defender's Study Bible Notes
4:3 sea monsters. The Hebrew word here translated “sea monsters” is tannin, also translated “whales,” and “serpents,” but most commonly “dragons” in the King James Version. Modern versions sometimes render it by “jackals” or “wolves” or “crocodiles.” This confusion is because the tannin, whatever they were, are now extinct. In the sixteenth century and earlier, however, accounts of dragons were still so widely known and reliable that scholarly Bible translators saw no problem in identifying the tannin as “dragons.” They knew that the Biblical accounts correlated realistically with the many similar records in early and medieval literature. Since the first dinosaur bones were discovered less than two centuries ago, the biblical accounts have been found to correlate with information paleontologists have provided about dinosaurs from reconstructing the many fossils of these once-abundant animals. However, the particular tannin in this verse seems to be a mammal, whereas most dragons seem to have been dinosaur-like reptiles. Possibly at least one kind of dragon/dinosaur was similar to the platypus, which has features of both reptiles (laying eggs) and mammals (suckling its young). Perhaps tannin, was understood as a generic term, applied to any large monster animal.
Paleontology has also revealed a number of exotic animals called mammal-like reptiles; many of these also were large and grotesque. In any case, dragons were real animals—probably dinosaurs or mammal-like reptiles or both—which did not become extinct until relatively modern times.