A slight tweaking to my own MissingNo. Interpretation.
It is animated to better show how it appears in motion, and I made an attempt to give it a "cry."
MissingNo. is best known for its unintentional appearances in the first Pokémon games. Caused by certain glitches, this glitchy Pokémon would sometimes appear in the wild. It typically caused graphical glitches and inadvertently multiplied the item listed 6th in your inventory. Typically its appearance would take the form of a scrambled pixel mess, though on some occurences it held other forms. Its form is entirely different in Pokémon Yellow.
This iteration bears a Stability Belt (or "StabiliBelt") to prevent it from spreading its glitching form from spreading uncontrollably, primarily keeping all contents within a certain radius. While it isn't entirely effective, it does keep things in a central location, and doubles as a mechanism with which MissingNo. can communicate both visually and audibly.
Aside from its spherical body, it also appears to bear a pair of free-floating hands. They can appear and disappear when needed, though it typically disables their presence by default. Interestingly, it was never able to do this in the past...
While it is unknown what thought process (if any) MissingNo. has, its behavior has been deciphered as confused, curious, and non-aggressive. However it will defend itself when attacked, and will use its glitchy abilities to strike, sometimes causing (assumedly) permanent damage.
Because it is an incomplete entity, its peak level appears to be at Lv. 75. It seems to be looking for something to "complete" it, whatever that may be.
Pokémon © Nintendo, The Pokémon Company, etc.
Art © 2011 Malamite Ltd.