The noun duckling designates a young duck, and the expression ugly duckling designates a person or thing that appears unattractive in its early stages, but develops into something outstandingly beautiful or successful.

This phrase occurs, for example, in an article about the Evening Standard’s New Homes Awards, which celebrate regeneration of industrial wasteland and run-down neighbourhoods all over London—article by Ruth Bloomfield, published in the Evening Standard (London, England) of Wednesday 3^rd August 2022:

As these regeneration zones start to mature, so new ones are springing up all across London, from Canada Water to Brent Cross, to Silvertown on the south side of the Royal Docks, where work on turning ugly ducklings into swans is in its earliest stages.

The phrase ugly duckling alludes to the story by the Danish author Hans Christian Andersen (1805-1875) of the cygnet hatched with a brood of ducklings, and despised for its clumsiness until it grew into a magnificent swan.

This story was first published under the title Den grimme ælling (i.e., The ugly duckling), in Nye Eventyr. Første Bind. Første Samling (i.e., New Fairy Tales. First Volume. First Collection – Copenhagen: C. A. Reitzel, 1843).

The first English translation was published under the title The Ugly Duckling, in Danish Fairy Legends and Tales. By Hans Christian Andersen (London: William Pickering, 1846).

The earliest occurrences of the phrase ugly duckling that I have found are as follows, in chronological order:

1-: From The Nelson Examiner, and New Zealand Chronicle (Nelson, New Zealand) of Saturday 23^rd December 1848—the adjective bipennated means two-winged:

Many of our readers, we dare say, have seen some of the pleasant stories told by Christian Andersen, a Danish author, whose writings have the singular felicity of amusing the young, while they instruct the old. One of these, the latest, we believe, which has been translated into our language, is called the “Ugly Duckling.” […]
The number of Ugly Ducklings in the world we dare say is considerable; but we fear the other class of bipennated beings—those who aspire, or whom accident places, above their natural level—is far more so. It sometimes happens, too, that the difference between the real bird and the counterfeit, is not readily detected. So it is among men. A reputation is often owed to accident—a character, which serves as stock-in-trade for life, is the result of no particular genius or ability, but is acquired by mere chance, and for a long time the world will almost wilfully shut its eyes to what it might at first have seen had it only taken a little trouble.

2-: From The Wellington Independent (Wellington, New Zealand) of Saturday 20^th January 1849:
—Context: The New Zealand Spectator and Cook’s Straits Guardian (Wellington, New Zealand), which was virtually owned by Robert Stokes (1809-1880), had accused The Wellington Independent of “miserable plagiarism”:

“Miserable plagiarism,” and this from a man who has throughout his political career, rendered himself a copyist of the sayings and doings of other men. A pamphlet containing an account of the “Ugly Ducklings” of Port Nicholson, would exhibit our learned friend in strange characters. The public opposer, but the private toady of one whom he frequently maligned—then as the opponent of Sir George Grey, but subsequently the Governor-in-Chief’s very humble and obedient servant:—and the chapter detailing his connection with the late Colonel Wakefield, would make known some curious facts—how a man can profess the greatest friendship, then the deadliest animosity; then a second friendship; another mixture of hatred, mingled with a desire to keep in with the “pocket;” ending with the most notorious toadyism. What a quantity of dirt some “Ugly Ducklings” do swallow; and when we reflect on the past career of the present friend of “Nomineeism,” we feel astonished at the quantity which our contemporary must have disposed of.

3-: From the following, by ‘an Old Hand’, published in The Southern Cross (Auckland, New Zealand) of Friday 8^th June 1855:

The Provincial Law Officer is rising in the opinion of the malcontents. For a while, with graceful, gentle wit, they dubbed him “Singular Crotchford.” At present, he is “the lynx-eyed lawyer.” There is no denying the fact: he is a lawyer; and could scarcely wish to be thought purblind. The compliment is the more delicate for being veiled.
For a while, his opinions were derided by those who were accustomed to Mr Swainson, and to the slovenly old routine: they are now examined with increasing interest. Their soundness can only be tested by time; but at least they show a care and a research which are unusual in New Zealand.
He may be likened to the ugly duckling of Hans Andersen’s tale, mocked at by the rest of the brood, because they could not discover that it was an incipient swan.

4-: From a letter to the Editor, dated Auckland, Friday 8^th June 1855, by one A. Gander, published in The New-Zealander (Auckland, New Zealand) of Saturday 9^th June 1855:

Sir,—After depriving us of the light of his wit for a fortnight, the “Old Hand” beams out once more this morning. Himself an “ugly duckling” mocked by a blind world, he has found a brother in the Provincial Law Officer, whom he unhesitatingly announces to be an “incipient swan” […].
[…] The Old Hand having encountered a man more crotchety and oblique than himself, sits down at his feet, and looking up admiringly, exclaims, “He is a lawyer”; if the “Lawyer” is as successful in doing for his clients as the Old Hand has been in settling those individuals who have permitted him to meddle in their affairs, his (Mr. R’s) reputation will soon be established, and those who “were accustomed to Mr. Swainson” will set a just value on the opinion of the “incipient swan” and of his callow mate the ugly duckling.

5-: From The New-Zealander (Auckland, New Zealand) of Wednesday 13^th June 1855:

The ”Old Hand,” after a well-turned introductory compliment, has announced his great discovery that that supposed “ugly duckling,” the Provincial Law Officer, is no duckling at all, but a genuine swan. We hope the discovery will end here, and that the learned gentleman may be fortunate enough to escape being hereafter proved by his dangerous friend to be neither duck nor swan, but a more foolish bird than either.

6-: From an article about the reappointment as Lord Chancellor of Ireland of the Irish judge Maziere Brady (1796-1871), published in The Dublin Evening Mail (Dublin, County Dublin, Ireland) of Friday 24^th June 1859:

Of Mr. Brady himself we do not wish to speak harshly. In private life we believe he is seen to great advantage; and we regret, for his own sake, that he has not been content to adorn that “post of honour.” Even as a lawyer he has always been painstaking and conscientious—“laborious” he has been in his profession, as well as “lucky,” but in the latter respect his career has been beyond all precedent or example. We well remember with what general astonishment the appointment of “Maziere Brady, Esq.,” to the office of Solicitor-General was received some twenty years ago. The public, nay, even the legal portion of the public, was at that time, it must be owned, in a state of deplorable ignorance as to the very existence of such a gentleman; and even amongst those who enjoyed the privilege of his acquaintance, there was a growing conviction that Assistant-Barristership might form no unnatural limit to the ambition of the future Lord Chief Baron and Lord Chancellor. “Voluit Fortuna jocari.” * But in justice to the estimate which was then formed of Mr. Brady’s attainments, we must admit that we have never since heard it stated that even under the influence, and with the prestige of those exalted positions, the “ugly duckling” ever did turn out to be a genuine swan.

* This is a quotation from Satire III, by the Roman poet Juvenal (Decimus Junius Juvenalis – ?60–?140 A.D.):

Ex humili magna ad fastigia rerum
Extollit, quoties voluit fortuna jocari.

translation from Hoyt’s New Cyclopedia of Practical Quotations (New York: Funk & Wagnalls Company, 1922), by Jehiel Keeler Hoyt and Kate Louise Roberts:

Whenever fortune wishes to joke, she lifts people from what is humble to the highest extremity of affairs.

7-: From an account of the annual commemoration at St. Mary’s College, Harlow, published in John Bull (London, England) of Saturday 6^th July 1861:

An amusing passage at arms between the Rev. H. L. Jenner and the Rev. C. Moody as to whether the college had at any period of its existence been an ugly duckling, in which the former gentleman had by far the best of the argument, convulsed the company.

8-: From Chapter 6 of The Woman I loved, and the woman who loved me. A story in twelve chapters, by Isa Blagden (1816?-1873), published in Frank Leslie’s Illustrated Newspaper (New York City, New York, USA) of Saturday 29^th March 1862:

Mannering was praising extravagantly the beauty of her sister.
“Yes. Nora is very handsome, and yet I remember, when a child, my poor mother was in despair about her personal appearance. She was very dark, the only one of us who was dark, and I believe the nurses thought she must be a changeling. But it was quite the case of the ugly duckling. It was marvellous how she improved as she grew up, till she was at last always recognized as the beauty of the Comptons.”

9-: From Incidents of the Parliamentary Session, published in The Huddersfield Chronicle and West Yorkshire Advertiser (Huddersfield, Yorkshire, England) of Saturday 24^th May 1862:

No political measure of any importance has been, or is to be, laid upon the table of the House of Commons, albeit during the last few weeks many very exciting skirmishes have taken place in the old arena between the old chiefs. The grand measure upon which the greatest battle has been done, has been the Re-re-revised and much reviled Educational Code. Hugging even to the death his ill-favoured and hopeless bantling, Mr. Lowe has come to grief more than once while fighting in its defence; while, alas! he has at last been obliged to succumb to the onslaught of foes and the desertion of friends, who, had he not yielded, with however bad a grace, would have left him alone with his ugly duckling, armless and hopeless. He has, however, avoided being kicked out of the ministry by a timely, if a graceless, submission.