Glitches from Pokémon, with sprite coloring changes.

From left to right we have:

MissingNo. or 'M from Pokémon Red and Blue. The only difference is that 'M evolves into Kangaskhan at level 0, and apparently learns Pound at level 136 [how people learned that I'll never know]. Starting moves are Water Gun, Water Gun [no that's not a typo] and Sky Attack.

MissingNo. from Pokémon Yellow. Reverts to level 1 when gaining any EXP. Starts with Pay Day, Water Gun, a blank placeholder and Bind. The blank placeholder hides itself and anything below it, preventing it from being selected. But it is auto-replaced when teaching a TM or HM move to the Yellow MissingNo.

????? or "Five Question Marks". Four slots are unused, where this glitch's data resides, in Pokémon Gold, Silver and Crystal. All four variants are different in learnset, type, Pokédex number and level up speed. Crystal's versions behave differently.

?????????? or the "Decamark". 386 slots are filled with real Pokémon, one's an Egg, one's a "bad EGG" and between the gen2 and gen3 Pokémon are 20-ish slots full of a Pokémon called "?". But there are 65535 possible slots for Pokémon in the 5th generation, the Decamarks fill the unused ones. Each with its own unique type, learnset, level up capabilities [some don't level at all] and abilities. Abilities and type may freeze the game, and some have even a different color pallette. Most have all stats set to a base value of "0", so they don't get very strong.