Are there any figures once thought to be mythical, whose historicity was later proved?

There were local tales of the Okapi that were believed to be exactly that, until a skull was found and brought to Europe. Wiki. A Scientific American article says that similar local tales existed for the, “…Kipunji Rungwecebus kipunji (discovered in 2006 following observations of a mystery monkey) and the Burmese snub-nosed monkey Rhinopithecus strykeri (discovered in 2010 following investigation of local reports about a “monkey with an upturned nose”).”
Also: 10 beasts that used be mythical. More dubiously or at least uneven, 10 mythical things that actually existed.

If the stature of Sancho the Strong is an example of a mythical attribute that was later found to be realistic, how about the deformity of Richard III?

For some decades the Richard III Society claimed that the stereotypical depiction of the hunch-backed king was Tudor propaganda, unsupported by the evidence. When, eventually, they found him in a car park in Leicester, the first thing that was apparent was his dreadful scoliosis. Despite this deformity he was still apparently able to take an active part in several battles as a warrior and as king.

John Henry might be an example. John Henry (folklore) - Wikipedia
The problem is that there are multiple candidates. But the earliest account from the 1920s seems most substantiated to me.

It wasn’t until 4364 AD that a cache of crumbling paper was discovered which proved both the Emperor Rosgwash II who joined with others to defeat the demon dragon-slayer Hister in the North ( not to be confused with Regnash I who crushed the Tribe of Flying Men and fought long wars in the Southlands like Karl the Great ), and the Dynasty of the Twelve Buxhes who brought fire to the lands of the disobedient easterners and were much loved by the gnomes who dug deep for the black, black Oil, were based on actual people.

True.

Ramses II being red haired is another thing.

I’d say it’s more likely to be inspired by the birth defect cyclopism, if it has to have a rational basis. And is it really true that Alexander brought the first word of elephants? Good ol’ wiktionary points out that Homer used the word elephas for “ivory,” and it is attested in Linear B (also meaning “ivory”), so it seems reasonable that they might have known of them prior to Alexander’s experience.

Yes, there have been some catches in the Med. The hypothesis at the time was that they were recent immigrants via Gibraltar.

Just after 2 minutes in. It’s hard to tell, but it doesn’t look like he had to leave the water. Octopuses are smart enough that maybe he was putting on a show for treats (also smart enough to know the plural is octopuses :D).

I’m skeptical, but presumably they had the skull and no skeleton. This doesn’t invalidate the theory, but the guy who proposed it was a big Nazi.

Thanks. Richard came to mind, but I wasn’t sure what the best example of what was confirmed. As you point out, a lot of it was shown to be false as well.

In 2505, new research will show how the un defeated Charlie Chaplin and his Nazi hordes.

Red hair wasn’t unknown in Egypt, so that isn’t way out there. It isn’t only found in Ireland (and even that country is only the 2nd highest group of redheads).

Take a look at a tusk-less elephant skull. The nasal cavity is huge, and located where you normally expect to see eyes. The eye sockets are tiny and located where you normally expect to see ears. If you had never seen an elephant, and had only the skull to work with, a one-eyed monster would not be an unreasonable reconstruction.

I’m not sure if this fits the spirit of the OP, but there was some speculation that Watergate informant Deep Throat was a composite character or an invention of Woodward and Bernstein. I don’t know how widespread that theory was. He turned out to be a real guy.

This is not true.

Not a hunchback at all.

*Richard III may be the most famous hunchback in history but it appears his deformity was mightily exaggerated by Shakespeare.
Scans of his skeleton show he only had a slight deformity that would have barely affected his appearance or prowess on the field of battle, say scientists.
The notorious description by Shakespeare of a ‘hunchback toad’, a Machiavellian villain suffering a twisted body, a limp and withered arm, was almost certainly an attempt to blacken his reputation, claim experts.
*

There is some evidence (two steles) that King David was real. Almost certainly he is mostly myth and legend, but based upon a real King.

The question then becomes, what does “based upon” mean? This is far more than a quibble. Using a remembered real name lends veracity to a story even if the story has no connection to the actual person. We know that the land of Egypt was a real place, but the Torah stories that are set in Egypt probably are sheer myth. If no slaves were used to build the pyramids then the rest of that particular story is probably equally mythical. Finding the name Moses in an Egyptian record would not make the person real or the story.

Even for the scholars who agree that the Tel Dan Stele (and possibly the the Mesha Stele) refers to a House of David can’t positively claim that any connection to the Biblical David has been established. As with most other ancient stories, we have far too little evidence for anything beyond ideologically biased claims.

He was far more deformed than I expected to see, having accepted the thesis of the RIII Society. He almost certainly needed to conceal his scoliosis with clever tailoring and special armour. But (as I mentioned earlier) he was still a formidable warrior.

It’s stretch to go from “the current thought is that the Pyramids were primarily built with tax levy skilled workers” to “No slaves were there at all”.

"In addition to the many unresolved arguments about the construction techniques, there have been disagreements as to the kind of workforce used. The Greeks, many years after the event, believed that the pyramids must have been built by slave labor. Archaeologists now believe that the Great Pyramid of Giza (at least) was built by tens of thousands of skilled workers who camped near the pyramids and worked for a salary or as a form of tax payment (levy) until the construction was completed, pointing to workers’ cemeteries discovered in 1990 by archaeologists Zahi Hawass and Mark Lehner. "

Slavery was indeed common in Ancient Egypt, as it was in most of the Ancient World.

Perhaps, indeed, the stonecutting and skilled work was done by tax levy skilled workers. That doesnt mean there weren’t slaves around to clean the latrines, etc. In fact this was undoubtedly so.

Except of course that the Exodus story never mentions the pyramids at all. It mentions building cities and storehouses and working with bricks and mortar and making bricks without straw, but nothing about pyramids. That’s something that was was an extra-biblical tradition–what else could the Israelite slaves have been building if it wasn’t the pyramids? It’s just not in the Bible, that’s all.

True.

The Bible makes no such claim. It expressly states that they built the garrison cities of Pithom and Ramses. Which were infact built around the time that the Hebrews are supposed to have been in Egypt.

It’s more likely the other way around - the blocks were probably quarried year-round, making that more likely to be a slave job (quarrying and mining were the worst slave jobs - mostly, slaves actually had it easier than peasants in Egypt, but not those slaves), Finished by dedicated block-finishers (again, more likely to be slaves) and then the blocks were moved in the off-season by masses of people - more likely to be the peasant levies. I mean, they’re peasants, they won’t have any masonry skills.