This post may be off topic to be in the Metal/Color Gens section, but I don't know where else to put it.
You can encounter Missngno. in DPP, it's just impossible to catch.
1st: I am unsure exactly how to do this, but on my R4 I went into Eterna Forest with some cheats on, and found TWO Missingo.s, it said "Wild !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! and dddddddddddddddddddddd appeared", or something like that, then my game froze. Also I was with Cheryl while that happened, then I kept having alot of game freezes until I turned all the cheats off.
2nd: When you team up with Marley, have a run through walls cheat on, as soon as you agree to travel with her, go through one of the walls and then WALK on some water (this probably won't work if you surf). When you go into a battle you will encounter a regular Pokemon and shiny Missingo. Your teamate will be a clone of yourself, and he will also have a shiny missingo. The wild missingo is impossible to catch because you cannot catch a pokemon while there are two on the field and by the time you can kill the other poke the wild missingno. will KO itself with struggle. These missingno.s will also be named !!!!!!!!!!!!!!!!!!!!!!!!!! and dddddddddddddddddddddddd and the like.
3rd: If you use an instant egg lay cheat with only one pokemon in the daycare it will lay an egg that will hatch into a bad egg, aka, missingno., if it hatches at all.