The MISSINGNO. most commonly encountered during glitches is a Normal/Bird-type Pokémon, whose sprite is a backwards letter "L"-shaped chunk of "fuzz". Bird is a beta type that was left in the game; it functions identically to Normal. The sprite results from the game treating non-graphical data as an image. This form of MISSINGNO. almost always knows Sky Attack and Water Gun; of particular note is the fact that it knows two Water Guns.

Other forms of MISSINGNO. use the fossilized Aerodactyl and Kabutops sprites from the Pewter City museum. The fourth form uses the sprite shown when one encounters a Ghost Pokémon in the Pokemon Tower without having the Silph Scope. These are actually separate glitch Pokémon that share a name; they can be distinguished both by their sprite and by their differing characteristics. "Fuzz" MISSINGNO., for example, uses a fixed moveset, whereas the fossil MISSINGNO.s' moves and stats change depending on the last Pokémon in one's party (among other things). The fossil MISSINGNO.s also tend to turn into RHYDON upon capture, if a player hasn't yet viewed their (empty) Pokédex entries (by, of course, capturing them).
Though they are all distinct, all known MISSINGNO. forms have several properties in common. They all share the Pokédex number 000. Encountering MISSINGNO. will increase the quantity of the sixth item in a player's inventory to above 128. (This is because the bit used to keep track of whether MISSINGNO. has been caught is also part of the byte used to track the quantity of the sixth item in a player's inventory.)

The sprite MISSINGNO.s use in the party screen is composed of random 8-by-8-pixel tiles shown on-screen. This means that it is often composed of chunks of terrain and NPCs. Viewing a MISSINGNO.'s stats screen causes a similar scrambling effect: most, if not all, all in-battle Pokémon and Trainer sprites become scrambled. Viewing the stats of a normal Pokémon fixes the problem. (making it more readable)