Okay. Here we go.
I'm just gonna quote one of Tamashii Hiroka's videos (thanks Eeveelution for the idea. xD) and say this.
Missingno. is a place holder which. Like '0' when you do numbers, to let the reader read the number correctly. The glitch was intentionally put in to keep the game going in a situation where the game doesn't know what to do. He/she/it won't appear out of the blue, since games don't have memory blanks. xD
Okay. Now, when you talk to that Old Man in the Old Man Glitch, what happens is that when the old man battles his test Pokemon, notice that the it doesn't say your name when you are battling him. It says 'OLD MAN'. Your name has to be stored somewhere, and they decided to store it on the coast of where Missingno. lays. (They can do that. Weird. xD)
Then you don't walk anywhere at all, when you fly off to Cinnabar Island... where your name's secret lies... :3
This is just one of my weird inquiries, something that the video doesn't explain is that when you open up the menu to choose your flying Pokemon, your name appears (to select your ID). Why does that appear, instead of what should be 'OLD MAN'? :o
Anyhow, now this is where you name comes in. Since your name is stored there, whatever you encounter there depends on your name. Not the usual Pokemon that roam there. It can be anything from Mewtwo Lv. 1 to Togepi Lv. 254. The odd numbers determine the species of Pokemon, while the even numbers determine the level. This determination is case-sensitive.
The game, depending on your name, will determine a value between 0 and 255. You encounter the Pokemon with this number on the coast! This is what you call the Pokemon's Hex Number. Every Pokemon has one. If you insert special symbols the game could possibly encounter glitch Pokemon in the high Lv. 200s!
Now, why Missingno. always appears is that your name ends with the 'EN' symbol when you choose your name (standing for End Name). This secretly appears at the end of your name, without the standard player knowing. This symbol is worth '50', which matches the number of Missingno. This will always appear in your name, therefore Missingno. will always be one of the Pokemon that appear.
Now, back to your question (xD) like Missingno., this was intentional. These are just unused sprites that appear in the game's data - they normally keep scrapped sprites and things in the game's data. It's like beta info that brings creepypasta fans their theories. xD
Now, to make the certain forms appear you have to insert the symbols in the specified places (which Knightofdragon has state) so these sprites can appear. The Fossil Aerodactyl and Kabutops aren't a bunch of pixels that amazingly formed boned Aerodactyl and Kabutops. They might have been used if you were looking at the displays in the Pewter Museum, but it may have been scrapped because it may have been inappropriate for little ones.
There are plenty of other things to explain which I won't do since I don't think their needed, but briefly when you catch it the 6th item in your slot becomes 128 and your Hall of Fame data will be glitched up.
Well, I've explained THOROUGHLY about this.
I really do hope I helped. :)
Source Thank you Eeveelution to linking her for me. Lol. xD I could translate her. :3