King cobras, despite their name are not considered “true” cobras. I understand that many critters (and even plants) have names that were given based on physiological similarities and not phylogeny (musk ox, electric eel, horseshoe crab, etc.). However, in the case of the king cobra, I haven’t been able to find what differences there are between it and the true cobras that warrant placing them in a separately grouping. Anyone???
King cobras eat other snakes almost exclusively. There’s that. Plus their size; they’re bigger. That’s all I can think of offhand.
Edit: I see from Wikipedia that King cobras make nests for their eggs.
Generally the word “cobra” has all the specifity of the word “hawk.” It describes an elapid snake that can spread a sizeable hood. The genus Naja is referred to as the “true cobras”, because it is oldest of the genus names and the original grabbag from which the others were gradually removed. But that doesn’t have any real scientific meaning - it’s just someone looking for a common name to distinguish a “true cobra” in Naja from a “spitting cobra” in Hemachatus. There is no body that regulates common names for snakes.
Snakes in Naja, Ophiophagus, Paranaja, Pseudohaje, Hemachatus, Aspidelaps and Walterinnesia have all been called cobras, loosely defined.
The mambas, Dendroaspis, which have a weak hood, traditionally have not been called cobras.
In 1999, Joe Slowinski, rest his soul, suggested that based on genetic studies that Ophiophagus may be more closely related to the mambas than the other “cobras”, which clustered nicely, but considered the issue unresolved. I’m out of date, so I have no idea if that has firmed up or has proven to be an artifact of the study.
However I suspect the original separation of Naja and Ophiophagus was based on very minor anatomical details. However I don’t have a genus description handy and haven’t been able to find one in a quick search.