It's time for another constellation, and we are moving on to one of the larger ones: the sixth largest of Ptolemy's constellations, in fact. This one represents something that definitely exists: the Po river in northern Italy, or the Istros of Hungry, which was located in the mythical northern land of Hyperborea. The ancient Hellenes called the river 'Eridanos', and that's the name of the constellation as well.
Eridanos (Ηριδανος) is a river God, and one mostly identified with the story of Hēlios and His son Phaëton, which I will come to in a bit. This association comes from a possible translation of the name: 'early burnt'. Phaëton's myth, in fact, is the only myth this river s identified with, but not even in a big way. Lets look at the myth first:

The most famous piece of mythology concerning Hēlios regards His son Phaëthon (Φαέθων), by Klymene (Κλυμένη). The story is told to us by Ovid, a roman poet. In it, Klymene boasts to Phaëthon that his father is the sun God Himself, and so, Phaëthon goes up to Olympus to confirm. To prove His paternity, Hēlios swears of the river Styx to give Phaëthon anything he desires. Phaëthon grabs this opportunity to demand of his father to let him drive his golden chariot the next time the sun rises.

Hēlios tries to talk His son out of it, claiming that not even Zeus would attempt to drive the chariot, as it is hot with fire and the horses wild and fire breathing. Phaëthon will hear none of it, and so Hēlios must let him get on. He rubs his son's body with magical oil that will protect him from the heat and as Eos and Apollon leave the gates, so does Phaëthon.

The four horses of the chariot--Pyrois, Aeos, Aethon, and Phlegon--sensed Phaëthon's weaker hand and became virtually unsteerable. First, Phaëthon drove them too high, and the Earth below cooled and the people suffered. Then, he flew too low and entire cities burned, lakes and rivers dried up, and even the seas were affected. Mighty Poseidon tried to stop Phaëthon, but had to flee from the heat. It was Zeus who threw His lightning bolt and killed Phaëthon. In some versions of the myth, Phaëthon's burning body landed in the Eridanos river. The following example is from Philostratus the Elder's 'Imagines', a Greek writer who lived in the third century AD:

"Now the youth is thrown from the chariot and is falling headlong – for his hair is on fire and his breast smouldering with the heat; his fall will end in the river Eridanus and will furnish this stream with a mythical tale." [1.11]

Hygenius identifies Eridanos with another river: the Nile. From his 'Astonomica':

"Some call this the Nile, though many call it Ocean. Those who advocate the Nile point out that it is correctly so called on account of the great length and usefulness of that River, and especially because below the sign is a certain star, shining more brightly than the rest, called Canopus. Canopus is an island washed by the river Nile." [II.32]

Eridanos is visible at latitudes between +32° and −90°, and best visible at 21:00 (9 p.m.) during the month of December.