In the first two or so issues, Beast was essentially a big dumb oaf who sexually harassed Jean Grey. As early as the third issue, he became a self-described bookworm who used a much more impressive vocabulary and was far kinder to people around him, and it was treated as if he had always been so. Interestingly, in a much later arc, the Beast, now blue and furry (which is also this trope, as some don't know he wasn't always this way), has his appearance restored to his original human form, but at the cost of his intelligence dwindling as he uses his powers. He eventually talks much like his original self.
Apocalypse's first appearances had him as a crime boss and then a Magneto-esque mutant supremacist, giving the same sort of speeches with more Bold Inflation during a time when Magneto had become the New Mutants' mentor. The "survival of the fittest" aspect of his philosophy emerged soon afterward and those other characterizations quickly fell by the wayside. His backstory and status as a rare but overwhelming threat came later still.
The original plan for the X-Factor comic (where Apocalypse was first introduced) was that the shadowy mastermind that X-Factor had been fighting against was planned to be minor Daredevil villain the Owl. When a new writer came in, they decided to create a new villain.
Magneto himself is a good example of this. The Stan Lee/Jack Kirby version of the character was just a straightforward villain with essentially no character depth. Chris Claremont gave Magneto a sympathetic backstory and changed him into a Well-Intentioned Extremist, and these changes have more or less stuck over the intervening decades. Unfortunately, post-Claremont writers and editors re-cardboardified Magneto to some considerable extent.
Rogue got her start as a villainous henchwoman working for Mystique, taking on and nearly single-handedly defeating the entire team of The Avengers with a bravado that would be out of place from the troubled young girl she was soon developed into. This is particularly strange when you consider that nearly all of Rogue's earliest appearances — including the aforementioned encounter with the Avengers — were written by Chris Claremont.
According to a tenacious urban legend, Wolverine's origin was intended to be that he was actually a wolverine mutated into human form. Len Wein himself, Wolverine's co-creator, has made it clear that this is just a myth. Old plans that were made are that his claws were originally intended to be built into his gloves, and Word of God states that he was planned to be about as strong as Spider-Man (i.e., at the low end of superhuman strength in the Marvel Universe, but still able to lift about 10 tons). He also started out as a thug with a bad temper whose main contribution to fights was rushing in recklessly and getting swatted aside to prove that the villain couldn't be beaten without teamwork. During The Dark Phoenix Saga, he Took a Level in Badass and never looked back. It's also surprising (considering how important it is to his modern characterization) how long it is between his first appearance and the first explicit mention of his Healing Factor (he says that he heals fast a little earlier, but it's in more of a "don't fuss over me just because I got myself beaten" context than an "I have a superhuman ability to heal" one).
And how about Charles Xavier himself? In the first twenty or so issues, he used his powers a lot more, reading minds whenever he pleased, communicating telepathically even to people in the same room, harboring a secret crush on Jean Grey, and mind-wiping several of the X-Men's foes in order to get them out of the way for good. While some later writers would portray Xavier as a Jerkass, it never reached this level. Early Professor X also had a close relationship with the US government and freely deployed the X-Men as government agents, a setup that would be unthinkable in the later "hated and feared" days.
Also, Iceman. Nowadays, he is mostly known as a funny guy. In the beginning, he would often attack his teammates during mission briefings. While this was probably intended to show him as immature (he was two years younger than the rest), it just made him look like a Jerkass and an idiot.
Glob Herman started out as the henchman/best friend to Quentin Quire, and was nothing more than a meatheaded bully. After spending about a decade Out of Focus, he finally became part of a regular cast in Wolverine and the X-Men (Marvel Comics), where he took a few laps through the Heel–Face Revolving Door and didn't see much character development. His appearance in Spider-Man and the X-Men was the first major step towards being a redeemed character, and his appearances written by Ed Brissonnote Old Man Logan (2016), Dead Man Logan, Uncanny X-Men (2018), Age of X-Man: Next Gen and New Mutants (2019) have fully reframed him as The Eeyore who cares deeply about his teammates and friends and seeks to solve conflicts with as little violence as possible (usually). This radical shift from his early appearances was given In-Universe reasons in New Mutants, where he explains that his father raised him to hate mutants, and turned the hatred onto him once his mutation manifested, leading to him hating himself for being a mutant until he realized that he was just continuing his father's hate.
Quentin Quire himself, in his original New X-Men arc by Grant Morrison, was a relatively nerdy and unpopular kid radicalized into violent and quasi-fascist politics. Later writers would transition him into more of a punk-alternative teenager who likes to project an outward image of rebellion against authority but is in practice more annoying than dangerous.
Julian Keller (Hellion) was originally portrayed in New X-Men: Academy X as arrogant, extremely overconfident in himself, and openly obnoxious to anyone he saw as beneath him, with his attraction to his classmate Wind Dancer being the only hint of a greater sensitivity beneath. Through a series of events (the death of his friend Tag, the loss of his hands, his relationship with X-23), he became a much more sensitive and sympathetic character — to the point that many writers seem to regard him as too "normal" to use much.
