Cetus is the Latin spelling of the name; the ancient Greek form was Kētos (Κῆτος), or Kêtos Aithiopios (Κητος Αιθιοπιος, Ethiopian Monster). It was the name of the sea monster sent by Poseidon as a favor to the sea God Nereus, who was insulted by the queen of Ethiopia, Cassiopeia, who boasted that either her daughter, or she, or both were equal or even greater in beauty than Nereus' children, the Nereids.
Cetus tormented the coast of Ethiopia, drowning many and wiping entire towns off of the map. Ethiopia's king, Cepheus, went to an oracle to find out how to stop the suffering of his people, and was told to chain his daughter to a rock on a cliff so Cetus could devour her. The royal family resisted, but eventually did what they were told. What happens next is beautifully told by Roman poet Ovid in his Metamorphoses:
"Chain'd to a rock she stood; young Perseus stay'd his rapid flight, to view the beauteous maid. So sweet her frame, so exquisitely fine, she seem'd a statue by a hand divine, had not the wind her waving tresses show'd, and down her cheeks the melting sorrows flow'd. Her faultless form the heroe's bosom fires; the more he looks, the more he still admires. Th' admirer almost had forgot to fly, and swift descended, flutt'ring from on high. O! Virgin, worthy no such chains to prove, but pleasing chains in the soft folds of love; thy country, and thy name (he said) disclose, and give a true rehearsal of thy woes.
A quick reply her bashfulness refus'd, to the free converse of a man unus'd. Her rising blushes had concealment found from her spread hands, but that her hands were bound. She acted to her full extent of pow'r, and bath'd her face with a fresh, silent show'r. But by degrees in innocence grown bold, her name, her country, and her birth she told: and how she suffer'd for her mother's pride, who with the Nereids once in beauty vy'd. Part yet untold, the seas began to roar, and mounting billows tumbled to the shore. Above the waves a monster rais'd his head, his body o'er the deep was widely spread.
[...] So when the monster mov'd, still at his back the furrow'd waters left a foamy track. Now to the rock he was advanc'd so nigh, whirl'd from a sling a stone the space would fly. Then bounding, upwards the brave Perseus sprung, and in mid air on hov'ring pinions hung. His shadow quickly floated on the main; the monster could not his wild rage restrain, but at the floating shadow leap'd in vain. As when Jove's bird, a speckl'd serpent spies, which in the shine of Phoebus basking lies, unseen, he souses down, and bears away, truss'd from behind, the vainly-hissing prey. To writh his neck the labour nought avails, too deep th' imperial talons pierce his scales. Thus the wing'd heroe now descends, now soars, and at his pleasure the vast monster gores. Full in his back, swift stooping from above, the crooked sabre to its hilt he drove. The monster rag'd, impatient of the pain, first bounded high, and then sunk low again. Now, like a savage boar, when chaf'd with wounds, and bay'd with opening mouths of hungry hounds, he on the foe turns with collected might, who still eludes him with an airy flight; and wheeling round, the scaly armour tries of his thick sides; his thinner tall now plies: 'Till from repeated strokes out gush'd a flood, and the waves redden'd with the streaming blood.
At last the dropping wings, befoam'd all o'er, with flaggy heaviness their master bore: a rock he spy'd, whose humble head was low, bare at an ebb, but cover'd at a flow. A ridgy hold, he, thither flying, gain'd, and with one hand his bending weight sustain'd; with th' other, vig'rous blows he dealt around, and the home-thrusts the expiring monster own'd. In deaf'ning shouts the glad applauses rise, and peal on peal runs ratling thro' the skies. The saviour-youth the royal pair confess, and with heav'd hands their daughter's bridegroom bless. The beauteous bride moves on, now loos'd from chains, the cause, and sweet reward of all the heroe's pains"
Cetus was visualized by the ancient Hellenes as a hybrid creature, with enormous gaping jaws and the forefeet of a land animal, attached to a scaly body with huge coils like a sea serpent. In some ancient drawings, Cetus comes out more comical than frightening, but the Cetus of myth was nothing to laugh at. Other visualizations of Cetus are in the form of a whale. This is mostly due to the latinization of the name; Cetus is the Latin word for the order Cetacea which includes the whales, dolphins, and porpoises.
The constellation Cetus is visible at latitudes between +70° and −90°, and best visible at 21:00 (9 p.m.) during the month of November.