Thats why you find items in Pokéballs; its the last item the pokémon in the ball holds..
Pokémon's Famous Missingno Glitch, Explained
Draven's Tutorial #11: The Missingno Glitch! (Green Version)
How To Do The Missingno Glitch In Pokémon Red and Blue 3DS
Over the years, people have battled and captured hundreds of Pokémon. And while everyone has their favorite, no pocket monster is as infamous as "Missingno." That's because Missingno isn't quite a Pokémon at all.
It all makes sense now! MissingNo. Pokemone glitch.
Pokemon Blue - Missingno Glitch - YouTube
Pokemon Green Walkthrough Part 60: The Missingno Glitch!
Missingno Art via Reddit user HorizoNxBringeR #Pokemon
12 MissingNo Glitch Pokemon Large Minky by SteelfeatherWorks