Seraphim - winged serpents (Book of Enoch)

According to the Jewish Encyclopedia the Seraphim are a class of heavenly beings, mentioned only once in the Old Testament, in a vision of the prophet Isaiah (vi. 2 et seq.). Isaiah saw several seraphim, their exact number not being given, standing before the throne of Yhwh. They were winged beings, each having six wings—two covering their faces, two covering their feet, and two for flying. The seraphim cry continually to each other, "Holy, holy, holy, is the Lord of hosts: the whole earth is full of his glory" (vi. 3).

The seraphim are frequently mentioned in the Book of Enoch (xx. 7, lxi. 10, lxxi. 7), where they are designated as δράκονες ("serpents"), and are always mentioned, in conjunction with the cherubim, as the heavenly creatures standing nearest to God. In Rev. iv. 6-8 four animals are pictured as standing near the throne of God; each has six wings, and, as in Isaiah, they sing the "Trisagion."
The older Babylonian concept of the seraphim is of serpents, as the name implies. Among many peoples of antiquity serpents played an important part in myth and folk-lore. For instance, there were Tiamat in the Babylonian legend of the Creation, and the Uræus serpent in Egypt. Consequently, since the Jews shared the superstitious ideas of surrounding nations in other respects, it should not be a matter of wonder if they adopted this notion as well... In this connection the names "Dragon Spring" and "Serpent Pool" (places in the vicinity of Jerusalem) are worthy of being noted. A brazen serpent brings relief from the effects of the bite of the fiery serpents (Num. xxi. 9 et seq.) which Yhwh sent among his disobedient people in the wilderness. Isaiah (xiv. 29, xxx. 6) speaks of fiery, flying serpents and dragons; and a brazen serpent, Nehushtan, stood in the Temple at Jerusalem, and was an object of worship until the time of Hezekiah, who destroyed it as being idolatrous (II Kings xviii. 4 et seq.). The worship of Nehushtan was plainly a remnant of ancient superstition, and was reconciled with the worship of Yhwh by connecting Nehushtan with the scourge of snakes in the wilderness and the rescue from them (Num. xxi. 9 et seq.). Therefore the theory seems possible, even probable, that the seraphim have their counterpart in the flying serpents of Isaiah (comp. also II Esd. xv. 29). It is only natural that these winged guardians of Yhwh's throne were soon ranked as higher beings and invested with the human form or with some features of the human body; and it was because of the very fact that they were adopted into the Yhwh cult that they were, in process of time, ennobled and spiritualized. Jewish Encyclopedia