The Marvel Universe is home to many powerful heroes, but it is not able to call one of their most powerful by his actual name. The Masked Raider is one of Marvel's earliest heroes, but he was previously known by a different name: the Masked Rider. How this name change came to be is a complex story of comic book companies folding, rights being purchased, and rights being transferred. Comics with the same names can lead to legal troubles for both parties.
Back in the early days of pre-Marvel, cowboy pulp comics were extremely popular. Martin Goodman - the publisher of the company that would eventually become Marvel - wrote his own cowboy pulp novel in order to cash in on the popularity of the Lone Ranger. Martin's character was named "The Masked Rider" and was published by Ranger Press. When Goodman moved on to comics - specifically, the comics company that would eventually have one of the most iconic logos in history - he brought the Masked Rider along. The Masked Rider even appeared in Marvel Comics #1, but under the name the Masked Raider.
The Masked Raider - as Marvel now calls him - made an appearance in 2019's Marvel Comics #1000, a fitting return for the character who appeared in Marvel Comics #1. But why the name change? According to CBR, Marvel sold the rights to the Masked Rider name in 1940 to Standard Publishing, which was publishing their own Masked Rider comics. Standard Comics continued publishing Masked Rider comics until well into the 1950s before going out of business and being sold to Fawcett comics (which also went out of business). Some non-Marvel entity out there holds the rights to Masked Rider and can use it, but for the meantime (at least until Disney finishes ironing out the legal rights for every Marvel character) Marvel will continue to publish Masked Rider stories under the name Masked Raider.
In his current form, the Masked Raider has moved on from his cowboy roots. While he still has the western aesthetic and carries two guns, he wears the Eternity Mask, a mask made from the same material that Eternity itself is. The mask gives him the ability to mimic powers and "cosmic awareness," an ability that allows him to see anywhere in the universe that he wants to see. Wearing Eternity on his person definitely makes him a powerful being in the Marvel Universe, but the complexities of the rights of creators in relation to their comics creations and publishers means that the Rider is the Raider - at least for now.
The early days of the comics industry were a bit like the Wild West, so to speak, with many different publishers putting out a variety of comics with many overlapping themes and even characters. Consider how some of the earliest superheroes have similar abilities. Tracking down the rights holder for the Masked Rider (in order to change his name to Masked Raider) may not be at the top of Marvel's priority list. However, characters that are not big hits now may become massive hits later on, as is the case with Daredevil, who Frank Millar revived. It's possible that the Masked Raider may become a hit in upcoming years, especially with his new appearances in the Defenders comics, and Marvel may want to make the return to the character's original name.
Source: CBR