Formalhaut is called the "Fish's Mouth" star because it is supposed to shine on the mouth of Piscis Australis, the Southern Fish. This Fish is represented in the star-maps as holding open its mouth in utter contentment while a long line of small, glittering stars wash down in a stream of water. This water flows from the Urn of Aquarius, the Water-bearer, who stands just below the head of Pegasus, the Winged Horse. The Urn is formed by a little Y of 3rd and 4th magnitude stars, and, to the Egyptians, was an emblem of the rainy season.
"Rolls from Aquarius' vase a limpid stream
Where numerous stars like sparkling bubbles gleam."
—Aratus.
The Southern Fish must not be confused with the two tied fishes of the constellation of Pisces, known as the "Northern Fish," which lie to the east of the Waterbearer's Urn. This region of the sky has been called the "Sky Sea," for not only does it contain three fishes but also Cetus, the Whale, Delphinus, the Dolphin, and Capricornus, the Sea-goat.
The three fish and the Seagoat were placed in the sky to commemorate the adventures of the Goddess Venus, the little God Cupid and the God Pan with the terrible fire-breathing monster Typhon. When Typhon appeared with a deafening roar, shaking his head and belching fire, the immortals fled in a panic before him, and when Venus and Cupid and Pan jumped into a river, they were transformed into two fish and a Sea-goat.
The constellations which border the "Sky Sea." All of these stars are not in view at the same time.