Satan is considered a (former) Seraph because he was made of fire;
'The
angels beheld the likeness and image of God in Adam and they fell down and
worshipped him and gave him glory as the likeness of God. But Mastêma, which
is Sataniêl, saw the honour that was given to Adam and he
was greatly
jealous. He stiffened his neck before the Lord and said unto the angels: "it
befits not us to worship Adam, for he is virgin (parthenikos) of earth; but
we
are angels (formed) of flames of fire
and honourable, (made) from the elements (hyle pl.) of fire, while he is earth
from the earth." Coptic encomium on Michael, attributed to Chrysostom, (Paris
131(1),43)
From W.E. Crum, "Texts Attributed to Peter of Alexandria" Journal of Theological
Studies 4 (1903) p. 396-97 n. 3.
This would also go for the other Archangels who could appear to be made of fire;
And they took me to a place where they were like burning fire, and, when they wished, they made themselves look like men. 1Enoch 17.1
Seraph is also the name of a snake in Israel with a very fiery bite which gave rise to the idea that the angel seraphs also were snakes.
In the Bible Satan is called a Cherub.
Thou (Satan) wast the anointed cherub that covereth: and I set thee, so that thou wast upon the holy mountain of God; thou hast walked up and down in the midst of the stones of fire. Eze 28:14
A Cherub is a creature that watches over something or someone, nations, the world or even larger districts in the heavens; a guardian. Cherubim are Assyrian or Acadian in origin. The word, in Acadian, is Karibu and means "one who prays" or "one who intercedes". In ancient Assyrian art, the cherubim were pictured as huge, winged creatures with leonine or human faces, bodies of bulls or sphinxes, eagles, etc. They were usually placed at entrances to palaces or temples as guardian spirits. In the Bible two Cherubs are placed on the Ark;
Make one cherub on the one end, and one cherub on the other end; of one piece with the mercy seat shall you make the cherubim on its two ends. Ex 25:19
So the Archangels including Satan once, were flaming angels; Seraphs and protecting spirits at the same time, which is why they also were called Cherubs.