I think the best single example might be Alexander von Humboldt (1769-1859).

Andrea Wulf’s excellent The Invention of Nature is essentially a book-length answer to this question. The German naturalist Alexander von Humboldt was a super-celebrity in his day and for decades thereafter, but barely remembered now. His funeral was the largest ever accorded to a private individual in Germany. More than a hundred animal species are named after him, in addition to mountains or mountain ranges on every continent, various plants and minerals, more and more than a dozen towns and counties throughout North America. The centennial of his birth attracted some of the largest mass demonstrations in the world, including a crowd of 25,000 in Central Park alone. Humboldt was even briefly considered as the name of the state we call Nevada.

Why Humboldt was so universally acclaimed and why he has subsequently fallen into such obscurity comes down to the universality of the principles of natural science he established. Humboldt was an explorer and naturalist in a time when the natural sciences were still in their infancy, often seen as the lesser spin-offs of pursuits like philosophy and literature. Humboldt travelled the world and developed ideas that are so fundamental to our understanding of nature that it is at times surprising that anyone needed to come up so the them. Namely, Humboldt was the first to popularize the idea that similar environments tend to produce similar forms of life. That is, there is a reason why the flowers found in northern Sweden, southern Patagonia, and the Tibetan Plateau tend to have certain similarities, even though these areas are far apart and the plants in question are not closely related. Humboldt was the first to conceive of what we would now call ecotopes- the idea that coniferous forests in Oregon and Korea have more in common with one another than either do with their neighboring plains.

Humboldt fell out of popular consciousness in part because these ideas have become so fundamental to our understanding of nature that their discovery can seem inevitable. Few people would build a statue of, for example, the first person to say the sky was blue (while he was not the first to say this, Humboldt did often measure the blueness of the sky with a device called a cyanometer). His ideas were so successful that it fell to later naturalists such as Charles Darwin to make the controversial leaps of reasoning that embed them in our cultural memory today. Moreover, anti-German sentiment in the early 20th century led the English speaking world with its vast global reach in publishing and opinion-making to disfavor Humboldt and privilege its own naturalists.

Today, Humboldt is little known outside of Germany, and even there his memory is often overshadowed by his brother Wilhelm, the celebrated philosopher for whom Humboldt University in Berlin is named.

Source: Andrea Wulf, The Invention of Nature.

How about the Roman Dictator, Lucius Quinctius Cincinnatus? He was sufficiently famous that he was the namesake for the city of Cincinnati and was a historical analog for George Washington ("The American Cincinnatus" or "The Cincinnatus of the West.") He was popularly known for two thousand years as an idealized exemplar of a righteous, effective defender of the republic and a humble, serious, simple farmer. But recently, when Boris Johnson compared himself to Cincinnatus returning to his plow, people seemed largely confused by the reference. (Evidenced by this Washington Post article: In his final speech, Boris Johnson compared himself to Cincinnatus. Who?)

Cincinnatus was a Republic-era Roman general and statesman who was among the most famous to assume the position of Dictator. While he was neither the first or the last to do so, he's notable for having been made dictator, successfully performing his duty to the Republic, and then immediately relinquishing power to return to his farm. And for doing so with remarkable success. And for doing it twice.

At the time, certain crises --- usually military in nature --- could be resolved by temporarily handing over absolute power to one person for a year, who was expected to do the job and then step aside allowing the consuls and the Senate to resume normal governance. Cincinnatus was appointed dictator in 458 BC to fight off an invading Italian tribe and made short work of the task. Six months later, the combatants were made to "pass under the yoke" in a humiliating ritual of their defeat. Then, again, almost 20 years later he was recalled to the position to quell an attempted coup to possibly install a king in place of the Republic. Three weeks later, Cincinnatus returned to his farm with the leader of the uprising dead.

Raphael used to be considered the greatest artist in the west, and was the gold standard against whom all artists were measured. It was widely considered among most people and artists that European art experienced a decline after Raphael's death, until it gained a resurgence after 1800 with the emergence of the Neoclassical movement, and later movements such as the Pre-Raphaelites, during this century Raphael was repeatedly criticized and scrutinized until his status as the canon of art withered away.

To quote a letter from the artist Mme. Elisabeth Vigee Le Brun, considered by many to be considered the greatest portrait painter in her lifetime, to an unnamed critic;

"Sir: I understand that in your work on painting you speak of the French school. As, from what is reported to me concerning your remarks, I gather that you have not the least idea of that school, I think I must give you some information that you may find serviceable. I presume, in the first place, that you do not attack the great artists who lived in the reign of Louis XIV., such as Lebrun, Lesueur, Simon Vouet, etc., and Rigaud, Mignard, and Largilliere, the portrait painters. As for the artists of the day, you do the French school the greatest injustice in rating it by its achievements of thirty years ago. Since then it has made enormous strides in a branch totally different from that signalising its decline. Not, however, that the man who ruined it was not gifted with a very superior talent. Boucher was a born colourist. He had discrimination in composing and good taste in the choice of his figures. But of a sudden he stopped working except for the dainty chambers of women, when his colouring became insipid, his style affected; and, this example once set, all painters tried to follow it. His defects were carried to the extreme, as always happens; things went from bad to worse, and art seemed irretrievably destroyed, Then came an able painter, called Vien, whose style was simple and severe. He was appreciated by true art-lovers, and regenerated our school. We have since produced David, young Louis Drouais — who died at Rome, aged twenty-five, just as he seemed to give promise of becoming a second Raphael — Gerard, Gros, Girodet, Guerin, and a number of others I might cite. "It is not surprising that after criticising the works of David, which you evidently do not know at all, you do me the honour of criticising mine, which you know no better. Being ignorant of the English language, I had not been able to read what you wrote about my painting, and when I was told, without being given the particulars, that you had abused me soundly, I answered that, however much you might disparage my pictures, all the worst you could say of them would be less than I think. I do not suppose that any artist imagines he has attained perfection, and, far from any such presumption on my part, I have never yet been quite satisfied with any work of mine. Nevertheless, being now more fully informed, and knowing that your criticism bears principally on a point that appears important to me, I believe my duty is to repudiate it in the interest of art. "Patience, the only merit you allow me, is unfortunately not one of the virtues of my character. Only, it is true that I am loath to leave my work. I consider it is never complete enough, and, in the fear of leaving it too imperfect, my conscience makes me think about it a long time and touch it up repeatedly. "It seems that my lace shocks you, although I have painted none for fifteen years. I vastly prefer scarfs, which you, sir, would do well yourself to employ. Scarfs, you may believe me, are a boon to painters, and had you used them you would have acquired good taste in draping, in which you are deficient. As for those stuffs, those eloquent cushions, those velvets, to be seen in my shop, it is my opinion that one should pay as much attention as possible to all such accessories. On this point I have Raphael as an authority, who never neglected anything of this kind, who wished everything to be explicit, to be rendered minutely — that is the language of art — even to the smallest flowers in the grass......"

Souvenirs de Elisabeth Vigee Le Brun

To quote some passages from the memoirs of the artist Joshua Reynolds;

"Those pretenders to Painting think the whole art lies in making things natural . If that were the case, how many Raphaels has Holland produced ? What I would endeavour to settle is the point to which the painter is to direct his attention, to give him an idea of what art is by the example of the Great Masters; for young painters , as well as connoisseurs , are some times puzzled in seeing a picture, in which there is nothing of what we call natural, preferred to another where there are satins, silks, jugs, etc., which deceive the sight"

J. Reynolds, one of the greatest British painters, was supposedly inspired to become an artist, on reading Richardson's treatise on art and architecture, in which he longingly wishes for the future emergence of a "British Raphael";

"Richardson no doubt expected the appearance of an English Raphael; and Reynolds, no doubt, hoped , and resolved, if possible, to fulfil such an expectation. H e told Malone that Richardson's treatise so delighted and inflamed his mind, that Raphael appeared to him superior to the most illustrious names of ancient or modern time"

Memoirs of the life of Sir Joshua Reynolds: with some observations on his talents and character

The famous Frieze of Parnassus, 4 sides circling the Prince Albert Memorial in England, had been sculpted as a homage to the greatest Poets and musicians, artists, architects and sculptors who lived, forming a "canon" of history's greatest. On the Frieze containing the artists, Raphael is seated in a throne in the middle.

Raphael is by no means obscure today, but most people, when asked about the greatest artists of the Renaissance, would probably recall Da Vinci or Michelangelo. The posterchild of the renaissance has become Da Vinci's Mona Lisa (‎Lisa del Giocondo), where in reality Da Vinci was obscure for most of the centuries after his death, and experienced a revival and resurgence in the 20th century, while Raphael faded from being the face of western art into the background.

This happens regularly in opera, most notably with Meyerbeer. Giacomo Meyerbeer (1791-1864) ranked among the most-performed opera composers in the nineteenth century (Vienna state opera archive) and was a reference point in other operas (eg. in Die tote Stadt by Korngold, 1922) as an obvious standard of the repertoire. He was a celebrity of the first rank during his lifetime and his music remained in the public consciousness for several decades thereafter.

By 1890 a turning-point had been passed, however. Wagner began his career a disciple of Meyerbeer and learned much from him, but in his style and dramaturgy split from and ultimately superseded his progenitor in his mature operas. His diatribes, most notably Das Judentum in der Musik, also damaged Meyerbeer's reputation. Ultimately Wagner's operas were the ones to replace Meyerbeer's in the repertoires of the major houses, most clearly the Met, whose decision around 1900 to become a German opera house meant it would play Wagner, not Meyerbeer (who wrote French opera, though he was German).

Meyerbeer's music was essentially forgotten through the course of the twentieth century, and while the occasional production takes place, his place in the repertoire now is a pale shadow of what it was.

Galen, known to his contemporaries as Claudius Galenus, was a Greco-Roman philosopher and doctor. Galen wrote a truly enormous^^(Note) number of works on medicine, the human body, and the proper development of spiritual and physical balance. He upheld the "four humors" theory of medicine, which stated that humans contained four substances (blood, black bile, yellow bile, and phlegm), which needed to be kept in proper equilibrium to maintain health. It is debatable whether he pioneered many of the theories and ideas he is known for, or was simply passing along knowledge that was widely believed at the time. During his lifetime, he was a medical celebrity, regarded as the best physician in the Mediterranean world. He died a little while after 200 AD, but his works lived on well after him. Due to his prolific output and high status among his contemporaries, he became the best known of the ancient doctors and medical philosophers.

For more than a thousand years, Galen's works were the reference for all questions of medicine in Europe and much of the Middle East. It was not until certain Muslim writers (such as Abu Bakr al-Razyi, known to the West as Rhazes) began to produce advanced medical textbooks in the Islamic Golden Age that alternative theories began to proliferate and compete with Galen. He completely dominated the philosophical space for medicine in the whole Early and High Middle Ages. Even al-Razyi wrote in his book Queries about Galen that "It grieves me to oppose and criticize the man Galen from whose sea of knowledge I have drawn much. Indeed, he is the Master and I am the disciple. Although this reverence and appreciation will and should not prevent me from doubting, as I did, what is erroneous in his theories." Eventually, Galen's ideas fell out of widespread use or were updated with new evidence, but his influence lingered well into the early modern era. Up until the Victorian age, the four humors were taken seriously as a medical theory.

Nowadays, Galen is remembered, of course, but his central ideas have little to no influence on the field of medicine. He was not a "celebrity" in the traditional sense, but he was a cultural touchstone in Europe and the Middle East for well over a thousand years. Any educated person would have known his name and his work. That is very far from true in the 20th and 21st centuries.

Speaking anecdotally now, I would be surprised if even 5% of people in most countries could tell you anything at all about Galen, though they might recognize the four humors. He is certainly much less well known than other ancient writers such as Socrates, Plato, Aristotle, and Marcus Aurelius. I would say he is a good example of what you were asking for.

^(Note: he really did write a lot, the Stanford Encyclopedia of Philosophy states that we have "21 volumes of roughly 1000 pages each—a total of more than 4 million words" from Galen.)

Sources:

Hankinson, R. J., "The man and his work", The Cambridge Companion to Galen, Cambridge University Press

Garcia Ballester, Luis. 2002. Galen and Galenism: Theory and Medical Practice from Antiquity to the European Renaissance.

Oswei Temin, Galenism: Rise and Decline of a Medical Philosophy, 1973