Well the reason is because, indeed, that they were doing alot of experimenting. If you checked out a hex-chart of the Pokemon in the 1st gen games, you will notice the Pokemon are really out of place. Rhydon being the first Pokemon ever programmed into the game's memory, which is why some glitches will turn in Rhydon....the games will try to fix the mistake by doing the next logical thing, start at the beginning. Also between the real Pokemon are Missingno.s. It is likely that they planned putting other Pokemon in the game, because if you look closey at a hex-chart, you will notice that some spots where Missingno.s and real Pokemon are seem a bit suspcious. Like how there is 1 Missingno. gap between Electabuzz and Magmar, could be showing signs of maybe them possibly to be a trio with another Pokemon. A R/B hex-chart can be viewed
here. Also the reason why some Pokemon evolve into Kangaskhan is because it's the second one programmed in the game. Infact almost all of the evolutions of glitches are some of the very first Pokemon programmed into the game's memory (Nidoking, Spearow, etc.).
Now for the reason of glitch Pokemon, there are 151 real Pokemon in the 1st generation. The game is only able to hold data for 255 Pokemon, so what happened to the other slots? Well this is where glitches come in. In Japanese Red and Green (not sure about Japanese Blue) very few glitch Pokemon that you could catch and use in battle were aviable. Mainly the Ghost and Fossil Missingno.s, and I think a few others. However when the graphics for the games were changed and the game given the complete English alphabet and put into English.....this altered the data for the glitches. The reason the sprites are so pixelated is because these are most likely the remnats of the old sprites, so they use these sprites as their sprites. Glitch Pokemon tend to have strange behavoir because well, they're glitches. Strange evolutions and movepools suggest that these Pokemon were likely destined to be something else, like how most people thought that Missingno. could of been a Kangaskhan pre-evo because it evolved into Kangaskhan. Others (like LM4) are more like a glitchy clone of another Pokemon. LM4, for example, is a glitched Poliwrath clone, because it has the same Pokedex number and learns some of the level-up moves of Poliwrath. Some glitches (again, LM4 being a good example) make strange noises for cries. LM4 will not make a cry during battle, but if you check his stats while on water...he makes the cry of a "Super effective hit". On land he will play a slow (non-glitch though) music (I find it slightly soothing) that stays on even after you check his stats....I beleive this to be beta-music of sorts. Other will use the "Got a key item" sound in battle.
The very first move programmed into the games was "Pound" and after Struggle (the last move programmed in), there is a glitchy version of Pound....then glitch moves follow onward. These glitch moves also changed, due to name changes and some having different animations (Thunderbolt looked different in Japanese Red and Green). Most just cause random and unpredictable things, some are actually clones of real moves (it's always Explosion though), or a glitchy version of an already exsisting move (like TM28, which is Explosion but the user doesn't faint!

). Alot of glitch moves just mess up the game or make it freeze.
Glitch trainers are basically...well trainers who use lots of glitches for Pokemon or glitched Pokemon. They tend to be a little dangerous though, I don't know much more about them seeing as how I didn't do alot of experiments with them. They most of the time have glitchy sprites.
Beta information left in, like the ????? item that is really the Surfboard....suggesting that one time during the early programming you used a surfboard instead of riding a Pokemon. Infact the Surfboard is the 7th item programmed into the game!! Also who can forget when Gulzar found HM06 Nickname? For those who don't know, HM06 made it so that you could pass through walls....probably used by testers to skim through the game quickly. Also there has been beta information found on Masterballs actually being able to be purchased! They cost alot though. Inbetween the Master Ball (yes, thats right! The very first item programmed in!) and the Max Elixer (last non-glitchy one) there are a total of only 3 beta items, 2 of them named ?????. The 3rd one (programmed a bit later) only says that Oak says it isn't time to use it right now. The 2nd is a beta Pokedex, which could actually be used in battle! It was programmed into the game early, when you use it however it causes glitchy effects (like the HP bars turning into bold letters) so that could be why the scrapped the idea.
Well, that sure was a long post! I hope I helped lots of people learn many things!