The title of this post was meant to be attention-grabbing. Bear with me, please.
Please note that I'm not maintaining these four creatures designated for special service of God are anything but angels. They seem to constitute a special type of angel, naturally, but I hold to the Biblically orthodox view that they were directly created by God, just as all the other heavenly beings in God's service were created. I don't hold the (nutty, IMHO) view that Ezekiel 1 describes a UFO coming to Earth with alien creatures on board. I don't believe alien intervention is in any way required to explain events of the past, including the events of the Bible.
But still...it's not surprising at all that one of the faces is human, right? Human beings are created "in the image of God," whatever that means exactly (I think it points to a spiritual truth rather than a physical one). Human beings are the focus the story of the Bible: the creation on the sixth day, the fall into sin, God showing Himself as one of us in the form of Jesus, human beings featured surrounding the throne of God in heaven after the end of this age.
So where do the other three faces come from? Clearly these are creations of God as much as mankind is, but what is particularly special about lions, eagles, and cattle? Why not bears, doves, and sheep? The answer most theologians give rests in symbolism. The four faces stand for four somethings, perhaps the four gospels. Each face represents some aspect of truth.
Certainly this is possible. Even though I believe the Seraphim really exist, God could have chosen the faces for this unusual creation of His for symbolic purposes. But I would feel better about this interpretation, if all the faces were animals. But one face is human and human beings are not primarily a symbol of anything. Humans exist because God created them to. They surround the throne of God because He wants them to be there, rejoicing in everlasting fellowship with Him.
What if God created other beings like us, that is, spiritually in His image, needing redemption, or perhaps having never fallen into sin? Creatures capable of understanding Him, intelligent in the same way human beings are? These creatures could possibly inhabit other planets in our universe, or altogether different universes. Technically such creatures would be aliens, though I don't mean the term "alien" as it is usually used in science fiction--to describe beings imagined to have been created elsewhere by random evolution.
What if a story featured a future humanity with interstellar space travel, who voyage out in search of intelligent alien life, only to discover a sum total of three other intelligent species? And one such species have faces like cattle, one like lions, and one like eagles? And all of them have stories of creation, fall into sin, and redemption running parallel with humanity's story recorded in the Bible?
It'd make an interesting story setting, at the very least...