Hyginus, Astr (De Astronomia) 2.10 CASSIEPIA:
Euripides and Sophocles and many others have said of her that she boasted that she excelled the Nereids in beauty. For this she was put among the constellations, seated in a chair [?] (original Latin).
Ovid, Met (Metamorphoses) 4.668-71:
Leaving innumerable nations behind, below and around him, he came in sight of the Ethiopian peoples, and the fields of Cepheus. There Jupiter Ammon had unjustly ordered the innocent Andromeda to pay the penalty for her mother Cassiopeia’s words (original Latin).
ApB (Apollodoros, Bibliotheke [Library]) 2.4.3:
For Cassiepea, the wife of Cepheus, vied with the Nereids in beauty and boasted to be better than them all; hence the Nereids were angry, and Poseidon, sharing their wrath, sent a flood and a monster to invade the land. But Ammon having predicted deliverance from the calamity if Cassiepea’s daughter Andromeda were exposed as a prey to the monster, Cepheus was compelled by the Ethiopians to do it, and he bound his daughter to a rock. When Perseus beheld her, he loved her and promised Cepheus that he would kill the monster, if he would give him the rescued damsel to wife. These terms having been sworn to, Perseus withstood and slew the monster and released Andromeda. However, Phineus, who was a brother of Cepheus, and to whom Andromeda had been first betrothed, plotted against him; but Perseus discovered the plot, and by showing the Gorgon turned him and his fellow conspirators at once into stone (original Greek).
Ovid, Met (Metamorphoses) 5.1-235 (excerpts):
While Perseus, the brave son of Jupiter,
surrounded at the feast by Cepheus’ lords,
narrated this, a raging multitude
with sudden outcry filled the royal courts—
not with the clamours of a wedding feast
but boisterous rage, portentous of dread war…
But when he saw his strength
was yielding to the multitude, he [Perseus] said,
“Since you have forced disaster on yourselves,
why should I hesitate to save myself?—
O friends, avert your faces if ye stand
before me!” And he raised Medusa’s head…
At last, repentant, Phineus dreads the war,
unjust, for in a helpless fright he sees
the statues standing in strange attitudes;
and, recognizing his adherents, calls
on each by name to rescue from that death.
Still unbelieving he begins to touch
the bodies, nearest to himself, and all
are hard stone.
Having turned his eyes away,
he stretched his hands and arms obliquely back
to Perseus, and confessed his wicked deeds;
and thus imploring spoke;
“Remove, I pray,
O Perseus, thou invincible, remove
from me that dreadful Gorgon: take away
the stone-creating countenance of thy
unspeakable Medusa! For we warred
not out of hatred, nor to gain a throne,
but clashed our weapons for a woman’s sake.—
“Thy merit proved thy valid claim, and time
gave argument for mine. It grieves me not
to yield, O bravest, only give me life,
and all the rest be thine.” Such words implored
the craven, never daring to address
his eyes to whom he spoke.
And thus returned
the valiant Perseus; “I will grant to you,
O timid-hearted Phineus! as behoves
your conduct; and it should appear a gift,
magnanimous, to one who fears to move.—
Take courage, for no steel shall violate
your carcase; and, moreover, you shall be
a monument, that ages may record
your unforgotten name. You shall be seen
thus always, in the palace where resides
my father-in-law, that my surrendered spouse
may soften her great grief when she but sees
the darling image of her first betrothed.”
He spoke, and moved Medusa to that side
where Phineus had turned his trembling face:
and as he struggled to avert his gaze
his neck grew stiff; the moisture of his eyes
was hardened into stone.—And since that day
his timid face and coward eyes and hands,
forever shall be guilty as in life (original Latin).
Cassiope claimed that her daughter Andromeda’s beauty excelled the Nereids’. Because of this, Neptune demanded that Andromeda, Cepheus’ daughter, be offered to a sea-monster. When she was offered, Perseus, flying on Mercury’s winged sandals, is said to have come there and freed her from danger. When he wanted to marry her, Cepheus, her father, along with Agenor, her betrothed, planned to kill him. Perseus, discovering the plot, showed them the head of the Gorgon, and all were changed from human form into stone. Perseus with Andromeda returned to his country. When Polydectes saw that Perseus was so courageous, he feared him and tried to kill him by treachery, but when Perseus discovered this he showed him the Gorgon’s head, and he was changed from human form into stone (original Latin).
Cited in note 17 for p. 308 (incorrect no. is given for Perseus): Pseudo-Eratosthenes, Katast (Katasterismoi) 15 (Kepheus; with a mention of Euripides); 16 (Kassiepeia); 17 (Andromeda; with a mention of Euripides’ Andromeda); and 22 (Perseus) = Mythographi Graeci vol. 3.1, ed. A. Olivieri (1897), pp. 19-21 and 25-27.
When Perseus son of Danae and Zeus had come to Cepheus son of Belus and married his daughter Andromeda, a son was born to him whom he called Perses, and he left him there; for Cepheus had no male offspring; it was from this Perses that the Persians took their name (original Greek).
Berlin, Pergamon-Museum F1652: Late Corinthian black-figure amphora with Perseus, Andromeda and kêtos
Drawing from Daremberg and Saglio, Dicionnaire des antiquités grecques et romaines (1896 ff.), vol. 4.1, p. 405
Stavros S. Niarchos Collection, Athens (cited as “Hirschmann Coll, no #”): Caeretan black-figure hydria with Perseus and kêtos
A.L. Jaffe, “Sea Monsters in Antiquity,” Berkeley Undergraduate Journal of Classics 1.2 (2013), fig. 2
Note 19 for p. 308, Scholia for Aristophanes, Batrachoi 53 (Scholia Graeca in Aristophanem, ed. F. Dübner , p. 276)
I am Echo, the nymph who repeats all she hears.  It was I, who last year lent my help to Euripides in this very place. But, my child, give yourself up to the sad laments that belong to your pitiful condition (original Greek).
Basel, Antikenmuseum and Ludwig Collection BS 403: Attic red-figure calyx krater with Perseus and Andromeda
Image courtesy of Antikenmuseum
As a great ship with steady prow speeds on;
forced forwards by the sweating arms of youth
it plows the deep; so, breasting the great waves,
the monster moved, until to reach the rock
no further space remained than might the whirl
of Balearic string encompass, through
the middle skies, with plummet-mold of lead.
That instant, spurning with his feet the ground,
the youth rose upwards to a cloudy height;
and when the shadow of the hero marked
the surface of the sea, the monster sought
vainly to vent his fury on the shade.
As the swift bird of Jove, when he beholds
a basking serpent in an open field,
exposing to the sun its mottled back,
and seizes on its tail; lest it shall turn
to strike with venomed fang, he fixes fast
his grasping talons in the scaly neck;
so did the winged youth, in rapid flight
through yielding elements, press down
on the great monster’s back, and thrust his sword,
sheer to the hilt, in its right shoulder—loud
its frightful torture sounded over the waves.—
So fought the hero-son of Inachus.
Wild with the grievous wound, the monster rears
high in the air, or plunges in the waves;—
or wheels around as turns the frightened boar
shunning the hounds around him in full cry.
The hero on his active wings avoids
the monster’s jaws, and with his crooked sword
tortures its back wherever he may pierce
its mail of hollow shell, or strikes betwixt
the ribs each side, or wounds its lashing tail,
long, tapered as a fish.
The monster spouts
forth streams—incarnadined with blood—
that spray upon the hero’s wings; who drenched,
and heavy with the spume, no longer dares
to trust existence to his dripping wings;
but he discerns a rock, which rises clear
above the water when the sea is calm,
but now is covered by the lashing waves.
On this he rests; and as his left hand holds
firm on the upmost ledge, he thrusts his sword,
times more than three, unswerving in his aim,
sheer through the monster’s entrails (original Latin).