About MissingNo
MissingNo is one of the website's many novelty Pokemon, inspired by the famous MissingNo glitch in the first generation of "Pokemon" games. In place of an actual Pokemon image and name, a jumble of graphic artifacts and "MissingNo" -- short for missing number -- appeared when the glitch prevented the game from generating a proper result. The sprites for MissingNo in GPXPlus match the sprites it could have in "Pokemon Red and Blue". To obtain MissingNo in GPXPlus, you must browse around the website until a small image of an old man appears in the page's header. Click on the image to obtain a MissingNo egg, which you can then hatch into MissingNo. If you are lucky, you may also find a MissingNo that was abandoned by another player in the Shelter.
Evolving MissingNo
MissingNo doesn't evolve normally. Instead, it can change into any one of four different forms: a Glitch form, a Kabutops Fossil form, an Aerodactyl Fossil form and a Ghost form. The only restriction to changing forms is a five-day waiting period between form changes. To change your MissingNo to a different form, move it to your party and wait five days, then use the options drop-down menu on your Poketch to select a form.
Eviolite
Eviolite is a special item on the GPXPlus website that can be used to bypass the five-day waiting period for form changes. You may occasionally find an Eviolite while browsing the website; Pokemon may also hatch holding one or bring one back from the Underground. If you have unlocked the Shop's backroom, you may also occasionally be able to purchase an Eviolite there. Because Eviolite is consumed when used, you will need a total of three Eviolite to change your MissingNo from its Glitch form to the other three without having to wait.
Considerations
Keeping a single MissingNo in your party for the fifteen days required to change it to all of its forms may prevent you from hatching as many eggs or adopting as many Pokemon as you want. If this is a concern and you have access to enough Eviolite, spending the Eviolite in order to change your MissingNo to all of its forms immediately then removing it from your party may be preferable. If you are lucky enough to have found more than one MissingNo, keeping multiple ones in your party simultaneously will shorten the time period required to obtain all form changes but also lower the amount of available space in your party for the duration.