Nothingno

[Tehmuffin]

Anyone know which pokemon game this thing is in, and how to fight him?

[ChaosYoshi]

Do you mean Missingno.? There is nothing called Nothingno in Pokémon. If you mean Missingno., then it can only be found in the early Pokémon games Red, Blue, (Green in Japan?), and Yellow. It is a glitch Pokémon that can only be accessed through doing certain actions.

[GalaxyMario]

It's actually possible to encounter it in later games, it's just far more difficult and requires Action Replay or something. Anyway, how to get him in Red/Blue:

Spoiler: show

1. Talk to the old man in Viridian City and ask him how to catch a Pokemon.
2. Fly to Cinnibar Island.
3. Surf up and down the east coast. By that I mean the stretch of water that touches the land.
4. Eventually, you will encounter something. It might be a normal Pokemon at level 128 or something like that, or a Fossil, or a Ghost, or just a glitched block of pixels.
5. Check your bag. The item in the 6th bag slot has been multiplied by 128.
6. Look in your hall of fame. It's screwed up.
7. If you caught Missingno, it is advised that you release it immediately, or more screwing up can happen.

[Tehmuffin]

It's actually possible to encounter it in later games, it's just far more difficult and requires Action Replay or something. Anyway, how to get him in Red/Blue:

Spoiler: show

1. Talk to the old man in Viridian City and ask him how to catch a Pokemon.
2. Fly to Cinnibar Island.
3. Surf up and down the east coast. By that I mean the stretch of water that touches the land.
4. Eventually, you will encounter something. It might be a normal Pokemon at level 128 or something like that, or a Fossil, or a Ghost, or just a glitched block of pixels.
5. Check your bag. The item in the 6th bag slot has been multiplied by 128.
6. Look in your hall of fame. It's screwed up.
7. If you caught Missingno, it is advised that you release it immediately, or more screwing up can happen.

Thanks

[Karyete]

Actually, Missingno is possible to get in Diamond/Pearl*, but sadly, I cannot remember HOW to find it, exactly. Something to do with Surfing and Contests in the middle of the map. Goldenrod, Nibasa....Anyone?

*U mad, Platinum?

[*Emelia K. Fletcher]

Don't you have to go to the Safari then do some complicated saving before the above procedure?
Never mind, that was the Glitch City one.

This is an explanation for all n00bs.

When surfing on the coast, Game Freak programmed there to be a legit place to catch Pokemon. Unfortunately, they forgot to program WHAT Pokemon are there. Talking to the Old Man sets some sort of Pokemon counter to the Old Man's name. Doing that and then going to the coast = MissingNo or 'M.

[Sethster]

If you guys want a further explanation on MissingNo., go to http://pokemon.wikia.com/wiki/Missingno.
Here's some info:
In the early Pokémon video games, the programmers had to use variables to refer to different Pokémon by number. Variable sizes must be powers of two. The smallest variable they were able to use was the size of one byte—that is, capable of holding any value from 0 to 255. (The next smallest size could only hold 0 to 127, which would not have been enough for all 151 Pokémon.) Because there are only 151 legitimate Pokemon in Generation I, this left 104 slots left unoccupied. 39 of these are occupied with glitches with the name of Missingno, 36 of which are occupied with the original sprite, 1 has the Kabutops fossil sprite, 1 has the Aerodactyl fossil sprite, and 1 has the Lavender Town ghost sprite.

Unlike most glitch Pokémon, whose names consist of data cobbled together from random locations, MISSINGNO.'s name is clearly a deliberately-added abbreviation of "missing number". This would seem to imply that MISSINGNO. was deliberately inserted as a placeholder or a removed beta Pokémon, albeit one with several odd quirks. The fact that 39 copies of MISSINGNO. exist (with each consuming its own "slot") would seem to support this theory.
Also MissingNo. and 'M are not the same pokemon, but have similar roles. MissingNo. only appears in Generation I, but other glitch pokemon have been found outside Generation I.
There is a glitch[3] (called the "Old Man trick") in Red, Green (Japan),and Blue that allows a player to battle nearly any Pokémon they wish, depending on the characters of their name. The player should start by viewing the Old Man's Pokémon catching tutorial in Viridian City. Immediately afterward, the player should Fly to Cinnabar Island and Surf on the east coast (the half-land half-sea tiles). They should not, at any point, swim onto a fully-water tile. Depending on the characters in the player's name, they may eventually encounter MISSINGNO. But in most instructions, it says not to catch MISSINGNO. because it will erase your save game. But this is not really true. In fact, MISSINGNO, may not delete it but cause game to freeze up and make the current save file unusable.
However, when performing the Old Man Trick, the wild Pokémon data holds the player's name, rather than the wild Pokémon available at the last location the player explored. When a wild battle occurs on Cinnabar's east coast, the game will read the player's name as wild Pokémon. The effect is that the text characters in the player's name will determine the species and levels of wild Pokémon on the coast. Not all available text characters correspond to normal Pokémon, meaning that this glitch may be used to encounter glitch Pokémon, MISSINGNO., included. Another thing to note is that most legitimate Pokemon can be encountered with this method, including Mewtwo, and even some "wild" Trainers that have glitched parties and may crash your game. Hunting these without experience with glitch Pokemon is inadvised.

This effect is relatively easy to exploit. The species are controlled by the third, fifth, and seventh characters of the player's name. The second, fourth, and sixth characters of the player's name determine the levels of wild Pokémon encountered via the Old Man Trick. The characters "w",[2][3] "x", and "y" will yield the Kabutops, Aerodactyl, and Ghost forms of MISSINGNO., respectively. If the graphics get scrambled, look at one of your Pokemon's Pokédex entries. The gameplay should perform normally.
U guys just got out-brained. I did reasearch!

[Zunar]

There's a guy who extensively breaks Pokemon Blue and part of that involves Missingno. It's a long article but it's very interesting.

http://lparchive.org/Pokemon-Blue/Update%2001/

[~MP3 Amplifier~]

I won't delete your posts as they are both contributing to the topic well, but in the future Sethster avoid bumping topics that have been inactive for a long time. Please read the rules if you are unsure about this. If it happens again I will have to issue a warning. Stickied topics don't count though you'll be glad to know.