Ok, I'm not responsible for anything happening to your game if you do this.
The Invinsible Shiny Bulbasaur is a literal legend accross the internet, but only people that hop to site to site to site will encounter how to get one. Luckily, I've been all over the web advertising this site: http://officialpokemonfanclub.webs.com
The Invinsible Shiny Bulbasaur, ISB for short, was originally created to entice naughty hackers that got Bad Eggs to hatch them. Of course, though a Bad Egg were made to ruin your game, it can also hatch into two things, which were both mistakes in the gaming software. One was the Togepi Egg. Another was the ISB.
The Togepi Egg would be obvious why, it only happens before you get your Togepi Egg in a game. The game code puts through that the first egg you get should be a Togepi Egg, so 1/100 times you would have a Togepi.
The ISB, however, was an accident that the programmers made. When designing Bad Eggs, since SOMETHING had to hatch they had to make a ISB hatch, because thats what they already put in Forums and News Updates. But since the ISB isn't actually a Bulbasaur, it usually does turn up in the wild as Missingno., and the fact that it does, in fact, not exist, is what makes game overload and a freezed screen. That is also why Bad Eggs in the wild can use Tackle and Growl, because the ISBhas the same move set as bulbasaur.
This fact makes ISB an indeed Legendary pokemon. You cannot obtain more than one, you can't trade it (Think of the Spikey-Eared Pichu), it's stats are extrordinaire (Actually, its stats are unlimited), and TALK ABOUT RARE! I have a code that gets you a Bad Egg on the very bottom, so go there if you want to try this for yourself. But, the ISB has the same info structure as Bulbasaur, which is why it's graphic is a shiny Bulbasaur, so it will try to evolve into Ivysaur.
Give it an Everstone.
If it evolves, it's doomsday for your game. No, not your save file, your game CARD. Kablooie! Bye, (Insert game name here)!
I'm going to leave this at that for now, but I will post more data about this amazing pokemon Missingno. in the near future.
PS To get a Bad Egg in the Hoenn region without a GameShark, go to the cave near the fighting type gym. There, you can battle and catch a Bad Egg. Then you can migrate it to your HeartGold game.
Here is the AR code for a Bad Egg (HeartGold, duh)