Marvel's Thor might be the most recognizable version of the Norse deity, further cemented by Chris Hemsworth's portrayal of the hammer-swinging thunder god. However, unlike most Marvel characters, Thor is a mythological figure, and as such, he's public domain. In order to trademark their version, Marvel has named the character The Mighty Thor.
Thor first appeared in Marvel Comics in Journey into Mystery #83 in 1962, though a completely different version of Thor had shown up a decade earlier in Venus #12. The most recognizable version, however, was created by Stan Lee, his brother Larry Lieber, and Jack Kirby. The character grew more popular, thanks in part to its legendary creative team, but this also created a challenge: how do you copyright a Norse god? Ultimately, Marvel was granted The Mighty Thor - the name of Thor's solo series at the time - as a recognized trademark by United States Trademark and Patent Office in 1970.
Of course, Marvel's trademark hasn't stopped other comic book versions of Thor from popping up from time to time in other publishers. For instance, Thor appears in Alan Moore's futuristic cop drama Top 10, depicted as a petulant and aggressive god who argues with the detectives looking into his kin Balder's death - and who, along with his fellow Norse, reenact the death of Balder at least several times a month. Jack Kirby himself used Thor for an earlier story in Tales of the Unexpected #16 in DC Comics, where a cowboy uses Mjolnir to get rich until Thor shows up and wants his hammer back. Thor also appeared several times in Neil Gaiman's seminal Sandman series.
Nor has the trademark prevented a superheroic Thor from appearing in a non-Marvel capacity. In 2011, Asylum Pictures, now famous for mockbusters as well as the Sharknado series, released their own version of Thor to coincide with the release of the first MCU Thor movie. The suspiciously-similar title for the low-budget affair was, fittingly, Almighty Thor.
Still, Marvel has benefited enormously from the trademark, as their character remains the most iconic version of the Norse deity. This has been helped largely by Hemsworth's portrayal of the character in the MCU, which has made the Marvel character a household name. While Thor's public domain status makes the thunder god fair game for virtually anyone, Marvel's long and storied history with the character, first established by Lee, Leiber, and Kirby, has become the most recognizable version all of pop culture. Thus, The Mighty Thor's trademark status might just be more powerful than Thor's hammer Mjolnir, as it's kept Marvel's version of the character alive in the hearts and minds of comic fans all this time.