I have all 151 pokemon in Yellow, then my friend stole it before leaving to another country. I still got ROM's... Anyway, the top posts after mine got me looking through my Missingno. databases.
Rumour:
Missingno. is another legendary bird, but they scrapped it.
Truth:
Not true.
How was Missingno. discovered?
In the first beta (until Viridian Forest), Nintendo staff were testing it. At that time, they noticed all wild pokemon were odd glitches. They fixed that problem, but the game had Missingno. and they actually named it to keep the game running, and not freezing. They decided that Missingno. should be appearing when the trainer changes (Old Man, anyone?) and then when people go to a place where there are supposed to pokemon, the trainer fixes and no Missingno.. However, there was a flaw in the plan. Somebody left the right side of Cinnibar Island without pokemon, but a surfing area, placing Missingno. there!
How did Missingno. get it's name?
Missingno.'s name came, because it's data was #000 and that's where the name came from. It's really straight-forward.
Did Missingno. appear in the US version?
Also, Missingno. was never found in the US version, but they still decided to name it that to keep the data from getting mixed up. It was only in the Japanese version when it was discovered, US version had nothing to do with it. US only found out that it's number is #000 and that's it.
What about Missingno.'s type?
The Normal/Bird type was an accident. Bird types were supposed to replace Flying types, but is Gyarados a Bird? It can't even Fly, but it's still a Flying type. Missingno. was never edited to be found and neither did it's type. Normal was type #1, so it's the first type there is. So, Normal was the first type existant, Bird was the normal Flying type.
How did Missingno. have these two types?
Okay... I know Normal is the first type in the game's data, but the Bird came from... Well, I don't realoly know why is it bird, but Missingno. is.
What about it's stats?
This is a real brain-buster, to which I can't really tell. I guess must have been random or something, I don't do it-detailed research, just Missingno.'s existence and anything else that shows up.