I think the story went something like this:
The maker of MGS and SSB know eachother quite well. Maker of MGS wanted to let them put Snake in Melee, but there wasn't enough time. Now that there is another SSB, Snake is going to be a character.
I could be wrong though because I forgot where I heard this.
Sorry I'm not good with remembering famous names of Japanese people.
Maybe they're friends now, but the way I heard it, Kojima just called and asked him if he could put Snake in the next Smash, but he had to tell him they where too late in the process to put Snake in. Otherwise, it's the same
He was actually supposed to be in Melee but Kojima was way too late for that one. So he's in this one.
And Snake belongs to Konami.
First appearance was on the MSX2, and eventually was ported on the NES. In fact, Metal Gear 2 was exclusive to the nintendo, and was apparently not released in Japan. There is more to metal gear than the playstation eye candy.