The X-Men formed as a mutant team that wanted to show the world that humans and mutants could live in peace. However, that proved to be a problem because humans hated and feared them, and evil mutants made them look bad at every turn.
When they first started, the X-Men not only had to fight public perception and humans who tried to hunt them down but also evil mutants and other Marvel villains to improve their public perception. The first batch of supervillains they fought included some of their most iconic rogues and some who fans have mostly forgotten.
Magneto
When Stan Lee and Jack Kirby created X-Men #1, the team had already formed, although Marvel wouldn't show that moment until a few years later in bonus stories. This allowed the X-Men to get into their initial battle instantly, as the first person they fought in comics was Magneto.
It is rare for a superhero's first villain to remain their most iconic, but this happened with the X-Men. Magneto became their most dangerous enemy and also was their greatest ally at other times. Magneto remained a villain that fans loved and the X-Men stayed connected to him for the last five decades.
The Vanisher
While Magneto was a very popular supervillain, most readers don't remember the second baddie the X-Men fought in comics. This was The Vanisher in X-Men #2. The Vanisher's mutant powers involve teleportation.
He worked as a common thief, using his powers to rob places while building a criminal organization. The X-Men beat him easily when Professor X altered his mind to take away his knowledge he was a mutant. While barely used, Vanisher is still around to this day as a member of S.W.O.R.D. In the end, Vanisher remains insignificant in the greater scheme of things.
The Blob
One of X-Men's most famous villains showed up in X-Men #3 with The Blob. This battle was interesting and revealed a little of Professor X's lack of scruples. The X-Men went to offer a place in the X-Men to Blob, who tried out and then decided he didn't want to join.
As a result, The X-Men attempted to force the issue and when he ran, they gave chase and attacked him. Xavier ended up wiping their identities from Blob's mind, but all that did was make him an enemy for life. Blob has remained a thorn in the X-Men's side, and like Magneto, was an early success story for X-Men villains.
The Brotherhood Of Evil Mutants
X-Men #4 introduced the world to the Brotherhood of Evil Mutants. Magneto founded the Brotherhood, the evil version of the X-Men, and versions of this team have remained a great threat in one form or another since this time.
The original members included Mastermind, Toad, Quicksilver, and Scarlet Witch. This group returned several times to battle The X-Men and even added The Blob in X-Men #7. The Brotherhood of Evil Mutants ended up as the principal enemies of the X-Men in the team's first movie as well.
Namor, The Sub-Mariner
In X-Men #6, the team battled a future legend in comics lore, Namor, The Sub-Mariner. In that issue, Namor, Marvel's King of the Seas, joined the Brotherhood of Evil Mutants. However, that would not last as Namor takes orders from no one.
Since that time, Namor has been a hero and villain, but his refusal to work for others remained a constant. Even more recently, the mutants asked Namor to take a seat in the Krakoan government and he rejected them.
Unus, The Untouchable
In X-Men #8, it was time for the X-Men to battle another lesser-known villain as they took on Unus, The Untouchable. This was his first appearance of the mutant, whose powers allowed him to generate an invisible psionic field.
Magento and the Brotherhood of Evil Mutants tried to enlist him and his test was to defeat an X-Men member. He failed but returned multiple times in the future to serve as a minor X-Men villain, although he is barely remembered and remains a mostly inconsequential early villain.
Lucifer
In X-Men #9, the mutants battled the Avengers for the first time. However, this was all part of an overall scheme by the parasitic villain, Lucifer. He was a villain from Xavier's past who at one time crushed his spine with a boulder, putting the X-Men's founder in a wheelchair.
The twist here is that the X-Men had to stop the Avengers from beating Lucifer because of a bomb that would trigger if he fell. While banished to an alternate realm, he returned a few times before finally dying at the hands of Archangel, finally giving Xavier a touch of revenge.
Maa-Gor
The X-Men met up with another Marvel hero in X-Men #10 when they went to the Savage Land and came across Ka-Zar and his sabretooth Zabu. The villain here was Maa-Gor, the last member of the Man-Apes in the Savage Land, and someone who wanted to kill Zabu.
This was Ka-Zar's first-ever Marvel appearance, and he helped the X-Men beat Maa-Gor and the Swamp Men he led. Maa-Gor showed up later in Ka-Zar's own title, but that was it for this villain.
Juggernaut
X-Men #12 featured the first appearance of one of the most powerful X-Men enemies, Cain Marko, The Juggernaut. This issue showed that Juggernaut was Professor X's stepbrother, but was not a mutant like Charles. Instead, Juggernaut got his power from a mystical gem.
Charles had left Juggernaut buried under tons of rock after he gained his powers, and he was back here for revenge. It took the Human Torch to help stop the villain, but he returned consistently through the years as a dangerous adversary, and sometimes teammate of the X-Men.
The Sentinels
X-Men #14 presented one of the most dangerous villains ever to fight the X-Men in comics. In this issue, Bolivar Trask made his first appearance and introduced the world to his Sentinels. These were the mutant-hunting robots that the government eventually used while trying to stop the "mutant menace."
In this issue, Trask has made his intentions to stop mutants known to the public. He created the Sentinels to do so, with 10 different versions of the robots introduced in this issue. The Sentinels caused years of pain and suffering for mutants and caused the Days of Future Past apocalyptic story arc.