They are the Babylonian symbols of the four fixed signs of the zodiac: ox or bull for Taurus (and earth), lion for Leo (and fire), eagle for Scorpio (and water), man for Aquarius (and air).
They can be thought of as representing the ancient four elements: earth, fire, water, and air.
They are mentioned in the Judeo-Christian Bible in the books of Ezekiel and Revelation. They have also been paired with the four evangelists and their books: Matthew, Mark, Luke, and John (although the pairing is not fixed).
They are shown on the Tarot card for the “Wheel of Fortune” in the Rider-Waite deck, and the card for the “World” in the Tarot of Marseilles.
Why these four creatures, and not others?