Kong is in the public domain, but King Kong is not - this is because the movie isn't in public domain, but the novel based on the movie but published first is in public domain. Hence why the MonsterVerse never calls him King Kong. And they used to be very litigious over the Kong part of the name too until the 80s when they lost their lawsuit against Nintendo over Donkey Kong. Pre-Donkey Kong lawsuit era Toho, yeah makes sense they wouldn't or couldn't risk it.