Kojima explains the Solid, Liquid and Solidus Snake names
On Twitter, Kojima took some time to explain his reasoning behind the name ‘Snake’, and the three adjectives, ‘Solid’, ‘Liquid’, and ‘Solidus’.
The reason I used Snake as code name in MG was snake was the most appropriate symbol of living thing that hides his presence, & sneaks w/o any noise. The reason I didn’t make any specific snake like cobra, anaconda, viper was because the protagonist is the player. The reason I use SOLID was to give opposite impression of soft image.
Like of all endings of the any series are, the appearance of strongest enemy was a must in MGS. It’s Snake who can surpass the Snake. Thus I brought about “clone”. SOLID vs LIQUID. That is MGS.
As to develop sequel, the 3rd snake was needed. Since both solid & liquid express state, means same true state. Naturally the next would be gas but gas snake is like gas human, not handsome name. So I borrowed from physics terms of “solidus/liquidus”. SOLIDUS is not state but implies the boundary of liquid and solid.
Source: Hideo Kojima Twitter

” Naturally the next would be gas but gas snake is like gas human, not handsome name.” bahaha I would have never took Solidus seriously if his code name was Gas Snake.
“Colonel Campbell, I’d like to introduce you to Solid Snake (brother of light), Liquid Snake (brother of dark), and um… Gaseous Snake (brother of flatulence).
Actually, I can kinda see it as Gaseous Snake being the brother of chaos seeing as the weather is hard to predict outside of climate averages… Maybe Plasma Snake as planetary guard because plasma is an outlaw? Or Supersolid? Or maybe Gaseous could be an insane fan of Solid, Liquid, and Solidus?
From now on you can call me Gas Snake 🙂
Mr.Gas what was your name before today?
Solidus
*And the plot thickens*
I was hoping he would use Viper. Such a cool name, though it was already use two times in the franchise.
One by Gene’s other codename then the Viper main antagonist from the Metal Gear Gameboy color game.
Gas doesn’t make any sense, and some crazy guy even mentioned Vapor Snake would be use.
No just no.
It’s true. This is not a handsome name.
Gas Snake would have been funny.