thetruemasterofgames : To be honest, I have no idea. I have heard that because the MissingNo glitch became so freaking famous, PokeFreak put him in the later games for the fans. But I can't confirm that is true. When using an emulator, you really shouldn't have anything to worry about. Just save state before you catch it. If it glitches stuff, just reload it. Since ROMs aren't cartridges that can be corrupted like that, the glitching isn't permanent if you just reload it. I wouldn't risk it on a real copy unless you look into it.

rcarter2 : i was asking because i play fire red and leaf green on cartiges if u do find out can u please pm me

thetruemasterofgames : You cant catch misingno. in fire red or leaf green it was not on purpess they put missingno. in red and blue it was just a glitch u cant catch it sorry

happy_timothy : so the only way to get it on fire red and leaf green is action replay? are u  100% sureabout

Yes the best glitchable games are pokemon red and blue fire red and leaf green are not glitchable (its hackable but its harder)

happy_timothy : thanks do u know the code for it so i can get missingno on there it's a useful Pokemon it can learn every move even special ones that only certain Pokemon are suppose to have like one of my friends taught it transform ( a move supposedly only available to ditto

Nope i dont know the code maybe google it or something

Mr moomoo : It was evolved into mewtwo because it was level 150-156 and mewtwo's number is 150 and not 151 because mew is number 151 thats why (i think) its evolved into mewtwo

It says Catching this pokemon would kill ur game...Well I caught him on my pokemon blue And my game died after a few weeks! DO NOT CATCH THIS POKEMON! Unless ur playing this on vizzed, but on ur actual Gameboy, Don't!

Well, I caught him before I don't remember it's level and it did know the move fly! lol

DwarvenKnuckleBuster : HUH dind the game work ever again or something ?????!!!!!

happy_timothy : No v-v I had to throw it away, Well, I play pokemon black now. I just don't know what happen to my pokemon blue when i caught him D; It just glitched up massively

happy_timothy :
If i remember corretly the type of missingo you get is based on the 3rd 5th and 7th letters in your name and the 2nd 4th and 6th letters determinate the level.

Heres a chart i found:

Type of pokemon:

A-Golduck
B-Hypno
C-Golbat
D-Mewtwo
E-Snorlax
F-Magikarp
G-Missingno.
H-Missingno.
I-Muk
J-Missingno.
K-Kingler
L-Cloyster
M-Missingno.
N-Electrode
O-Clefable
P-Weezing
Q-Persian
R-Marowak
S-Missingno.
T-Haunter
U-Abra
V-Alakazam
W-Pidgeotto
X-Pidgeot
Y-Starmie
Z-Bulbasaur
a-Missingno.
b-Missingno.
c-Missingno.
d-Ponyta
e-Rapidash
f-Rattata
g-Raticate
h-Nidorino
i-Nidorina
j-Geodude
k-Porygon
l-Aerodactyl
m-Missingno.
n-Magnemite
o-Missingno.
p-Missingno.
q-Charmander
r-Squirtle
s-Charmeleon
t-Wartortle
u-Charizard
v-Missingno.
w-Missingno.
x-Missingno.
y-Missingno.
z-Oddish

Level:

A-128
B-129
C-130
D-131
E-132
F-133
G-134
H-135
I-136
J-137
K-138
L-139
M-140
N-141
O-142
P-143
Q-144
R-145
S-146
T-147
U-148
V-149
W-150
X-151
Y-152
Z-153
a-160
b-161
c-162
d-163
e-164
f-165
g-166
h-167
i-168
j-169
k-170
l-171
m-172
n-173
o-174
p-175
q-176
r-177
s-178
t-179
u-180
v-181
w-182
x-183
y-184
z-185
(-154
)-155
:-156
;-157
[-158
]-159
pk-209
mn-210
?-239
P with a line in it (money sign)-240
X (multiplication sign, not letter)-241
.-242

The glitch itself is caused because of (or atleast i think)
when the old man shows how to catch the pokemon the game doesnt use your name so it stores it in the place where it isnt specified what pokemon there is aka the border between cinnabar and the water. Since they forgot to specify what pokemon appear there. Causing the glitch to happen. Im not entirely sure though
Missingno. always has water gun water gun and sky attack learned when you catch it.

reimarkable : WOW u know a lot about missing no. dude nice man

happy_timothy :
lol thx the chart itself though was not mine already knew the rest though :3

reimarkable : Thats actully what i taught too LOL but still nice job

Usually nothing happens to me when I catch it, but people say that it can ruin the game.

aaron500000 : Yes by some people it ruins there game but with some people no its based like reimarkable said: The its based on your name in the game. maybe if u call yourself Aaron then it will not glitch(maybe)

patar4097 : wrong there is another method to get yellows missingno. i know because i have yellow and encountered him by mistake. i was trying to find mew using the long range trainer glitch, and u no that guy on that bridge near laverder town, in the middle with the electrobs, i battled him, went back to the long range trainer, and it took awhile to the battle to work, knowing what missingnoxepert had said, i being to question , and he appered, scared like a noob, i shut my game off, then regerting it, for 1-2 days, to encounter him aging

kingvictory : True at lavender town use the man glitch (left of lavender town the entrance there is a man go to the menu just after u step for itthen fly then he ! and go to the left of lavender town there will missingno. (or mew) be it depends on the number of pokemons in your party and there nick-names or names u cant catch it at leaf green or fire red justat the original pokemon blue/red/(yellow)