i've been playing pokemon games since i was abouttt...11/12yrs old? somewhere around there. i honestly couldn't tell you exactly what got me into it; i remember catching glimpses of episodes from the anime on t.v when i was even younger, which was either reruns of the early seasons or then-current runs of one of the diamond and pearl seasons, but i don't remember going out of my way to try and keep up with it.
i didn't end up getting into it proper until a few years later. i must've just seen people talking about it online and finally got interested enough to pick up a game. i wanna say i saw black and white in a store and asked for it for xmas. i already had a ds that i played the absolute hell out of all the time so it wasn't an issue for my mother.

and ever since i first dug my claws into it, i've been hooked.

the typing system, the battling, the evolving, the mere concept of so many amazing creatures existing and willingly being our friends. there were so many! and they all looked so unique! some of them exploded or looked like actual ice cream while still being living, breathing animals! you could put them in your pockets! it was incredible.

my first journey was with black. i don't remember if i got white at the same time, or a bit later, but i had it too. black was definitely my main of the two though. i chose oshawott and got him all the way to samurott before i hit a wall and stopped playing. i think i chose tepig for white, but i did not get far at all, since i mostly played black. i also got black 2 later when it and white 2 came out but i remember absolutely NOTHING about what i did on that cartridge.
then there was soulsilver and heartgold. again, soulsilver was my main, and again x2 i do not remember a single thing about my heartgold cartridge. i didn't get very far on my soulsilver playthrough, and i've decided to spiritually continue it with my recent digital save. even named my cyndaquil Ash, just the same.
(yes, he was named after the character)
and then...the favorite child. pokemon y. i never bought x because at this point i knew that i was just going to play one and leave the other in the dust, so i figured i was better off not wasting the money. the new 3d modelled style was an astounding novel at the time, and the amie feature was so incredible. y was easily the game with my most hours put into it, as after beating the elite 4 i then set out to breed every single shiny i liked with a japanese ditto i acquired over GTS specifically for this task. i ended up hatching over one hundred shinies through my endeavers. fennekin was my starter, and it was also the game that i got my absol, Kaus. ♥
and, finally, the last pokemon game i bought was moon. i was absolutely smitten with lunala and rowlet before it had even released, and i'd say that it ties with soulsilver in second place for my favorite pokemon games. literally every single character was soooo charming and Hau is easily my favorite rival; no contest.

oh, and at some point i got alpha sapphire. no idea when. absolutely zero recollection of it other than i know without a shadow of a doubt that i picked mudkip.

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now, with my actual gameplay stuff out of the way...it's time to talk speculatve zoology!!!
i even have fun pictures to show examples ::]

one of my favorite things to do is think of the pokemon world in a more realistic light. not too realistic, mind--trying to envision it in a 1-1 comparison with real life just kind of ruins the fun of it more often than not--but natural things that happen anywhere and everywhere, like mutations, and hybrids, and variations in a species.
animals irl don't always look exactly the same; so why should pokemon??

admittedly my biggest fascination lies with color mutations. albinism, melanism, leucism, piebaldism--you name it! i personally feel like they would be considered on the same level as shinies, with different rarities within that single category.
for example, piebaldism would be one of the most common mutations you could come across(due to domestication factors), while albinism would be much more rare, even moreso than a shiny.

obviously, pokemon breeders would be able to try and breed for certain traits--and, thus, mutations--but just like in real life these efforts can very quickly lead to horribly sick and/or malformed pokemon. i imagine there would be a lot of pressure from official breeding organizations to make sure that anyone breeding long-term is striving for both looks and health in their pokemon.
and i don't think this would be a very unrealistic ideal for a majority of breeders, either; after all, the pokemon universe has proven itself several times over to be largely better than our current reality, and not just because friendly animals like absol and arcanine exist in it.

i like to think that the sprites we see in-game are a pokemon's 'standard' appearance; what would be considered 'optimal conformation' for a species. ones that would presumably perform better in contests and battles compared to their peers that don't conform to the standard, that sort of thing.

i also believe that the rarer a pokemon is, and thus the more unlikely a trainer is to own and breed it, the less likely it is to have not only variations but mutations as well. or at least, mutations would be much less documented than in more common species.
there would always be professors and other studious types who would strive to learn more in these circumstances, though, of course.

but, when breeders don't particularly care about sticking to a species' standard as long as it's healthy, or when wild pokemon do as they are won't to do...you naturally get variations! long, medium, or short fur/hair, longer or shorter snout, different tail lengths and shapes, all kinds of varied proportions in general--the possibilities are nearly endless.

especially when compatible pokemon of different species breed!! hybrids are easily another favorite concept of mine--as well as a very popular one within the community at large, obviously. for as long as the world of pokemon has existed, people have taken it and lovingly shaped it in their hands like conditioned clay.

now, back to the topic of variations--i actually cobbled together a little something yesterday to better visualize my concept.
i feel like certain pokemon would have more variations than others, like magikarp for example probably has TONS of variation and known + bred-for mutations, while others would have just a few, expected basics. like a certain dominant color being lighter or darker, or hue-shifted, etc.
in fact you can look at those exact examples below, using umbreon(and glaceon)!

as you can see, in this particular example umbreon have eight total color variations--three for their base coat, and five for their markings, all of which can be combined in literally any way.
as i touched on earlier, there are specific colors and combinations that would be considered the general 'standard,' or 'classic' appearance for a pokemon, while any other color combination might be considered 'undesirable' by particular groups. in reality, there isn't usually that much difference other than...well, the colors just being different!

i randomly chose a base and marking color for those three in the middle to showcase some possible variations that a breeder might experience.

and then, of course, comes the concept of hybrids! given how evolution works, especially in the case of those examples up there, some pairings would be more likely to have more 'obvious' tells of hybridization than others, with the offspring of different-species pokemon getting entirely random, potentially mishmashed, traits from their parents.
(which would also mean that, in theory, a baby pokemon could manage to only get traits from one parent, making them appear to otherwise be purebred even in the case of wildly different parents, excepting potential color standard discrepencies.)

going back to the above example, a glaceon + umbreon pairing would lead to baby eevees, of course...but they certainly wouldn't look like your average eevee. well, color-wise, at least.
as you can see the first eevee has the base coat of the glaceon, with the marking color of the umbreon, while the second eevee has the base coat and tail marking color of the umbreon, with a ruff colored like the glaceon's crest. they also have the eye colors of the glaceon and umbreon, respectively.
if evolved, regardless of which eeveelution they evolve into, their colors will very likely fade to something more 'standard' for the evolution. for example, if the blue and yellow eevee evolved into an umbreon, their yellow markings would stay the same, but their base coat would darken significantly, UNLESS special care was given to them in order to ensure that their original colors were kept throughout their evolution.

of course, color mutations will also lead to a pokemon having 'non-purebred' colors, but the two situations would visually be very different, each with their own tells, and as such it would be very easy to tell between a hybrid case and a color mutation case.

another thing related to variations that i would like to elaborate on, which i briefly mentioned above, is colors staying consistent throughout evolution.
i imagine trainers would need to be very good, and especially very specific, with grooming their pokemon in order to encourage them to keep their original colors as they evolve. naturally, they're inclined to change and warp as they grow, but with enough persistent care they can pretty reliably keep the colors they were born with.
(NOTE: this is specifically about variations and other expected colors, not color mutations. you wouldn't hatch a melanistic pokemon and then when they evolve suddenly they don't have mutated genes anymore LOL)

the only examples i have show shiny colors, as my original concept was shiny-specific variants. but, i mean, why the hell not apply it to non-shinies too, yknow?

again, as you can see, the usual (shiny) colors of the second and third stage of the line are on the bottom, while this theoretical snivy whose trainer raised them carefully to keep their colors would do just that across their evolutions; excepting serperior's white mask, bcoz there weren't any other colors to use from snivy's sprite, and i didn't have any particularly grand ideas aside from putting a 50% white layer over the underbelly color and calling it done lol. consider it a staple trait of the species affected only by other variations, if you please.

my og concept also included being able to have second or third stage colors on the first stage of a pokemon's evolution and etc., though that's only with the extra concept of 'pokemon with more than one stage can potentially hatch shiny in the colors of one of their later evos.'
purely as a mechanic for more variety, or for people who love the shiny colors of only one stage in an evo line but conceptually would also love to have the other stages in the same colors, too. it's definitely a fun concept to think about, especially in a collectathon gameplay sense.

and in terms of mutations, such as albinism and leucism and etc., well...they would look pretty similar to how you'd expect any other animal with a color mutation to look, just pokemon instead! though, considering they aren't 'just' your average old animal, i like to think that, while still having a mutation's distinguished look, they would also have their own little details for each individual.
for example a melanistic pokemon with originally light purple markings might have them turn a deep, royal purple, instead of just turning a dark brown or similar color like other areas of their coat would. i also like the idea of their eye colors changing, if only slightly, even if it might not make perfect sense for an irl animal with the same mutation. it's pokemon! you're allowed to have a little fun with it!

here's some mutated absols i made a little while back to show you what i mean.

keeping in mind these were originally made with game mechanics in mind, since personally i think that not nearly enough fangames get wacky enough with this kind of shit--in a broader, not-limited world, chimeric pokemon wouldn't be beholden to just being normal/shiny like shown here.
using my umbreons up there as an example, a chimeric umbreon could be black/harvest on one 'side' and chocolate/crescent on the other. again, literally SO many possibilities!

ermmmm i don't know how to really send this off LOL. uhh never let canon keep you from imagining cool things. for example i also like to think that legendaries can breed and they just have extremely low fertility, and that any pokemon can be 'male,' 'female,' or 'sexless' regardless of species. make it your own.