Gen 2 Eeveelutions
In Generation II, Game Freak introduced two new Eeveelutions, the Psychic-type Espeon and the Dark-type Umbreon. Most people were probably expecting perhaps a Grass and Ice Eeveelution before that, but I don't think many people were too disappointed since Psychic and Dark are "cool" Types, and their designs are generally considered to be very strong and on par with the originals, although notice they're lacking Eevee's furry collar as I mentioned before. Espeon and Umbreon are related to the sun and moon respectively, and despite there being both a Sun and Moon Stone that'd be perfectly viable to use in Generation II, Game Freak decided to have them evolve when they reach max Happiness, Espeon if that occurs during the day and Umbreon if that occurs during the night. Thus, if the player does not choose to evolve Eevee into its other forms and simply uses it in battle, eventually it'll probably try and evolve into one of these two, so perhaps you could say they're more "default" forms?
Despite popular belief, Ruby and Sapphire does indeed have a time system, although it's not visible like in Gold and Silver thus Eevee can indeed evolve into Espeon and Umbreon in those games, but Fire Red and Leaf Green lack the time system completely, meaning they're unavailable in the Kanto remakes. Game Freak literally locked out all the Generation II evolutions during the main game of FRLG, so it's no surprise they didn't bother with finding a way to let players obtain Espeon and Umbreon without trading, which was a major disappointment and one of the reasons I don't care for Fire Red and Leaf Green. I'm glad they didn't repeat this with HGSS and ORAS, instead letting players use the new evolutions if they so chose. In Pokemon XD, which also lacked a time system, Genius Sonority introduced two new items---the Sun Shard and Moon Shard---which allowed players to evolve Eevee into Espeon and Umbreon, and why Genius Sonority could think of doing that but Game Freak couldn't be bothered is a question for the ages.
However, most Generation III players likely obtained an Espeon and Umbreon through Pokemon Colosseum, where they were the Starter Pokemon of the protagonist Wes and pretty much the strongest Pokemon in the game. The rest of your team had to be hobbled together by rather weak Pokemon who had to also be purified first of the Shadow gimmick before they could live up to their true potential, but from the beginning Espeon and Umbreon can basically wreck anything they go up against and they're really the MVPs of the game. I don't care much for the Eeveelutions, and Espeon and Umbreon's designs don't do anything for me at all, but using them from the get-go in Colosseum was such a power-trip, and they're the first Pokemon I legitimately raised all the way up to Level 100 simply by playing through Colosseum and using them in every fight. I still have my original Espeon and Umbreon from that game today sitting in a special Box in ORAS, and I plan to keep them as long as I can. God, I loved Colosseum, one of the best Pokemon memories I have.
So, let's talk about Espeon and Umbreon.
#196 - Espeon
Psychic
Espeon as you can tell by its purple color-scheme is naturally the Psychic-type Eeveelution, and like most Psychic-types is known for its powerful Special Attack and Speed to counter its rather unimpressive physical stats. A Psychic-type Eeveelution in Generation I would've probably been very successful, but when Generation II rolled around, Psychic-type Pokemon were on the decline slightly and beyond that, Espeon had a rather plain move set that put it behind most of its Generation I counterparts. It mostly played support roles, Calm Mind + Baton Pass being a popular strategy, and in Generation V became more useful thanks to the Ability Magic Bounce, allowing it to counter all those annoying hazards that pop up in the competitive scene like Stealth Rock and what have you. Still, it seems its popularity was short lived, because in Generation VI it seems it's fallen back down to the lower tier. I imagine a certain Ghost/Steel Pokemon might be partially responsible for that? I imagine most players in Gold and Silver used Espeon, unless they rushed to get one of the older Eeveelutions, because most kids probably spent the majority of their playtime during the day hours, and thus Eevee evolved into it rather than Umbreon upon maxing Happiness.
I'll say it right here, but Espeon being the "Sun Pokemon" makes no sense. Nothing in its PokeDex mentions any relation to the sun, and its coloring and appearance make it seem much more like a creature of the night rather than something who'd appear in the daytime. I feel like they might've had another evolution method in mind for Espeon originally, but after making Umbreon they decided to connect the two of them to day and night, even though in Espeon's case that doesn't fit whatsoever. But what can you do? As far as Psychic Pokemon go, Espeon's what you'd expect, having the purple coloring commonly associated with them (although arguably yellow was their main color in Generation I), mystical eyes that show how intelligent it is, and a gem on its forehead which is probably the source of its power. Espeon is probably the one Eeveelution who I'm sure about what it's based on, as it's definitely the Nekomata, a yokai said to be born when a house cat lives long enough that its tail splits in two---you probably recognize the Nekomata thanks to the Yokai Watch mascot Jibanyan, who is also based on one. However, the actual Nekomata was a rather dark creature, originally it was basically a wild cat known to attack people, but most common depictions have it as being quite loyal and goodhearted, as seen with Espeon who is very loyal to its Trainer.
If there's any Eeveelution who is meant to be Eevee's canon evolution, I'd argue it's Espeon, although Sylveon recently has given it some competition which I'll explain in its entry. There's various reasons I get this impression, from Red having an Espeon in Gold and Silver, to its Japanese name, Eifie, being the closest to Eevee, and its general design. Plus I feel like if players don't go for any Eeveelution in particular, Espeon will end up being the one they likely get. Game Freak's never commented on which Eeveelution is the "canon one", and there probably isn't actually one they considered the natural evolution of Eevee, but sometimes I wonder if that was the plan with Espeon, to simply be the Happiness evolution in general regardless of time, but then they decided to make two new Eeveelutions. This might explain why Espeon's entries tend to talk about its relationship with its Trainer, while Umbreon's entries make it out to be quite feral, and not fitting a Happiness evolution.
Espeon's Signature Move is Morning Sun, fitting its status as the "Sun Pokemon", but once more nothing in its lore or appearance really jives with it having this ability. Actually, judging by its PokeDex which talks about how it's able to basically perceive the future and such perhaps Future Sight would've fit better, but Espeon actually didn't pick that up till Generation IV. Morning Sun isn't that bad, of course, better than Pin Missile. Anyway, I'd say in terms of popularity, I feel that Espeon is probably the most all-around popular Eeveelution due to looks, usefulness, and the impression he left thanks to being on Red's G/S team, although he was dumped for Lapras in HG/SS for some reason. Maybe because Espeon was associated with manga Red, and Lapras with anime Ash, who during Generation IV had become a literal God taking on Legendary Pokemon and thus they dropped the one Pokemon Game Red had who wasn't part of any evolution line he used?
Espeon had a somewhat important role in the Johto anime as being the main Eeveelution of Sakura, one of the Kimono Girls and the most important one in terms of plot importance, although she only appeared in two episodes---then again, Johto had very few reoccurring characters (Casey?), so that's pretty good overall. She later popped up in a Pokemon Chronicles episode with her Espeon challenging Misty to a Gym Battle, but hasn't been seen since. Espeon was also used in the Battle Frontier season by Salon Maiden Anabel as her final Pokemon in her rematch with Ash where it went up against Pikachu.
#197 - Umbreon
Dark
Umbreon's our first Dark-type Pokemon we've covered, excluding Mega Gyrados, since they were absent from Generation I and no Pokemon from Generation I were re-typed to Dark, nor did they evolve into a Dark-type in Gen II except for Eevee. Dark Pokemon were pretty odd in Gold and Silver, being pushed all the way back to Kanto for some reason, meaning if you wanted a Dark-type early on, Umbreon was your man. Umbreon's primarily a defensive Pokemon, allowing him to do what Dark-type Pokemon do best---play dirty and annoy the opponent. His move set is rather unorthodox compared to the others, and I remember in Colosseum he sort of played a backseat role for me while Espeon did all the work killing everything. So, for in-game players, Umbreon might've been a bit of a disappointment since stall strategies aren't that fun or time conductive when playing through the scenario, but in the competitive scene he is quite good due to his impressive defenses, a popular combo being Mean Look + Baton Pass to basically trap your opponent with an unfortunate match-up. I really need to learn how to play Pokemon beyond using all the fast, speedy attackers one day. But as one of the most common Dark-types, Umbreon was easily able to stick in people's minds, and he's probably right behind Espeon in popularity.
Umbreon's dark as fuck, bro. While Espeon had like nothing to do with the sun, Umbreon totally fits as the "Moonlight Pokemon", being pure black and covered with golden rings resembling an eclipsed moon. It's specifically said to have evolved due to the moon's "energy pulses", and at nights, its rings glow to strike fear into the hearts' of its foes. Yeah, Umbreon doesn't mess around. I imagine Umbreon is meant to represent "black cats", and other creatures of ill-omen from various folklore, but he may also draw from the bakeneko, another cat-type Yokai known for its mystical, monstrous powers, who I mentioned already as having inspired Meowth as well. Like Espeon, Umbreon looks fairly close to Eevee aesthetically, thus fitting his role as a more "natural" evolution, although the Happiness gimmick doesn't work quite as well with him as it does with Espeon. I do wonder if he was just gonna evolve with the Moon Stone, but suddenly Game Freak got super concerned about canon and dropped that idea.
If Espeon's Signature Move was Morning Sun, then Espeon's was definitely Moonlight, although it always shared it with a few Pokemon since Generation II and nowadays it feels a bit odd being "his move" due to it becoming a Fairy-type move, arguably the antithesis of Dark Pokemon. Don't be fooled by the name, though, because Moonlight is equally effective in sunlight and is virtually identical to Morning Sun now. Umbreon's PokeDex also talks about it being able to spray poisonous sweat from its glands or something, but I have no clue what move this is meant to reference as it doesn't learn any Poison-type moves naturally. The PokeDex is a weirdo sometimes.
Even though players of Pokemon Yellow were used to Gary having a Generation I Eeveelution, his Eevee ultimately evolved into an Umbreon, and was his signature Pokemon for awhile, although Blastoise and Electivire both took over that role, and Umbreon actually wasn't even used in the Johto League. Dawn's mother, Johanna, also owns one in Diamond and Pearl and it's also the first Eeveelution Virgil, the Eeveelution "Master", obtained and I believe was his Starter as well. Still, fuck Virgil.