Generation IV Eeveelutions
Generation III came and went without any new Eeveelutions, meaning Eevee fans had to wait until 2006 for new members---and after about a decade, the dream of a Grass-type and Ice-type Eeveelution finally came true! But like a lot of dreams, when they come true they're not always as glamorous or exciting as you thought they'd be, and for me when we finally got Leafeon and Glaceon, I felt pretty numb towards them. There's nothing bad about them, but they're basically what you'd expect Grass and Ice Eeveelutions to look like, and as someone who was never a fan of the Eeveelutions, they were easily the weakest of the Generation IV Cross-Generation Evolutions who I overall enjoyed. I don't think I'm alone as while they have their fans like all Eeveelutions, I feel they're far less popular than the Generation I and II Eeveelutions, and less so than Sylveon even as well.
Leafeon and Glaceon are "Location Based Evolutions", like Magneton to Magnezone, evolving at the "Mossy Rock" and "Ice Rock" respectively, which is a bit of an issue for them. Now, I enjoy the idea of location-based evolutions, but they have their immediate issues, such as requiring Game Freak to make said areas available in every game going forward, and for a Pokemon with multiple evolutions like Eevee, most folks will likely want to go with the other, easier Eeveelutions rather than tracking down the specific spots needed to evolve him into Leafeon or Glaceon. In Diamond and Pearl, like most Gen IV Evolutions, Leafeon and Glaceon were shoved to the post-game which feels odd since you'd think Game Freak would want to put them front and center. Platinum fixed this, but then Game Freak just didn't even bother with the locations in Heart Gold and Soul Silver, meaning Leafeon and Glaceon aren't available in those games, which is once more pretty dumb since Mystery Dungeon was smart enough to just create "Mossy Rock" and "Ice Rock" items to use for the evolutions, so why couldn't Game Freak? Black 2 and White 2 also fucked things up as despite adding Eevee to the Regional Dex, the Mossy Rock and Ice Rock are locked to post-game areas, meaning they're once more shoved to the end game again. XY and ORAS set things straight, but it's still pretty odd how little Game Freak seems to care about these two, and perhaps echoes the idea that their creation was just them "going through the motions" and doing a Grass/Ice Eeveelution because that's what the fans wanted.
But I don't mean to sound totally negative about them, so let's look at them individually in more detail.
#470 - Leafeon
Grass
Well, here it is, the Grass Eeveelution everyone wanted since Generation I, and I'm pretty sure "Leafeon" was the most popular speculated name for it behind "Grasseon". Since Game Freak couldn't just use the Leaf Stone though, Leafeon instead evolves when Eevee Levels Up around a Mossy Rock. You'd think the Eeveelution representing the Grass-type might be somewhat delicate, with high HP and a focus on a slower style of fighting, but nope, Leafeon is very physically oriented with high Attack and Defense and basically plays like Jolteon somewhat as the Eeveelution you might put at the front to lead the charge. However, being a pure-Grass Type Pokemon isn't exactly the best claim to fame, and Leafeon's fairly outclassed by many of his Grass-type pals who are a bit more versatile than he is.
While all the other Eeveelutions are basically still mammalian Pokemon, Leafeon seems to be more plant than animal, described as being a rather sedentary Pokemon despite appearances who soaks up sunlight and utilizes photosynthesis to live as opposed to foraging for food. And Leafeon's design reflects this quite nicely, its fur seamlessly transforming into plant-like material on parts of its body, and I wonder if eventually Leafeon will completely turn into a plant as its former nature as an Eevee completely disappears? Perhaps this could illustrate a dangerous side of Eevee's adaptability, its body being susceptible to being influenced to the point that Eevee has no control over what it becomes. Even though Leafeon doesn't do much for me personally, I'll say it looks far more imaginative than what most fan-drawings of "Grasseon" looked like, and I think the combination of the green and yellow looks really nice as opposed to making it solid green like fan depictions usually had it looking like.
Despite its high Attack, the PokeDex claims "it basically does not fight". Uh, except it clearly does and learns such attacks like Swords Dance and Leaf Blade, clearly showing it's naturally got an aggressive side. Leafeon doesn't really have a "Signature Move", although it sort of adopts Leaf Blade, which before it showed up was only learned by two Generation III Pokemon, but since Leafeon learned it quite a few Pokemon, including one of its Generation IV Cross-Generation buddies, have since picked it up. On the other hand, since Leafeon was the first Eeveelution to be introduced alongside Abilities, perhaps you could say it has a "Signature Ability", as it was the first Pokemon to have Leaf Guard as its primary Ability and greatly fits into its lore of being a photosynthesis-using namby-pamby as it protects it from status effects while Sunny.
The Diamond and Pearl anime was one of the first series, despite introducing new Eeveelutions, to lack an "Eeveelution Family Reunion" come to think of it, and it wasn't till Best Wishes we saw it alongside the rest of its friends. Leafeon didn't appear till the tail-end of Diamond and Pearl under the ownership of Dawn's Rival Zoey, and never really got a focus episode to itself, mainly just being used in battle. B-b-but it doesn't fight, right?
#471 - Glaceon
Ice
An Ice Eeveelution wasn't speculated about as much as Leafeon, due to there never being an item designed specifically for Ice-type Pokemon, but as the only Special-type remaining alongside Grass and Dragon, naturally fans expected one to pop up eventually. Glaceon evolves from Eevee when it Levels Up around an Ice Rock, meaning it tends to be available far later than Leafeon is since snowy areas typically fall at the very end of the game. Like many Ice-type Pokemon, it's got ice-hard Defense (but like that's much help with all their weaknesses), but also focuses on Special stats as well, having a powerful Special Attack. It's pretty flawed though, suffering from its Type being naturally pretty poor like Leafeon's Grass-type, and lacking in Speed, which seems to be the biggest flaw of Ice-type Pokemon and something Game Freak loves to do to them. It also apparently has somewhat of a reverse-Flareon situation, lacking decent Special moves to take advantage of its Special Attack.
Glaceon's design isn't as good as Leafeon's in my opinion, and there's not too much to say about it except it does a decent job at giving off an Ice-vibe, but I wish they got a bit more creative with it and it was literally made of ice in parts like how Leafeon has leafs growing from its body, rather than simply being cool colored. It sort of reminds me of Flareon in this aspect, not really sticking out among the rest of its fellow Type, but I guess that simplicity is probably what Eeveelutions aim for so I can't criticize it too much. Until Sylveon, I also think Glaceon was the most "feminine" of the Eeveelutions, despite being a 87.5% male species, due to its bangs that brings to mind a classic Japanese woman's haircut, I believe they call it the "Hime Cut", although most fans seem to see all the Eeveelutions, except perhaps Jolteon and Umbreon, as female generally it seems. I won't dwell on this point since it's a somewhat tricky topic, and there's another Pokemon coming up in Generation II that fits this topic better, but I think it's fair to say that in general, most Pokemon are coded as "male", or seen as such by the general populace, with the ones coded as female being the exception. But that's a cultural thing and may be different for you---for me, at least, growing up animals were all "he" unless they had a specific gender code saying otherwise, and I've found it hard to break from that despite being pretty open-minded when it comes to human gender-roles. You'll notice I tend to use "he" when talking about most Pokemon, and I really don't even try to, it just happens automatically when I type.
Lore-wise, Glaceon's pretty simple, and what you'd expect from an Ice-type Pokemon, freezing the atmosphere around it to summon a "diamond-dust flurry" (a 'secret' weather in Generation IV in fact that only happens on certain days), and forming ice crystal. It's bit like Vaporeon therefore, as both of them are not only able to adapt to their environment, they're able to influence it as well. Glaceon's body doesn't seem to be made of snow or ice, but it does have the ability to freeze its fur and fire it off like missiles---so, yeah, totally ripping that off Jolteon. Rather than giving it Pin Missile , I presume this is meant to represent Ice Shard, which was always shared with a lot of Pokemon so isn't that much of a "Signature Move", and also can't be put to much use by Glaceon as its Attack is rather unremarkable. Like Leafeon, it does seem to have a "Signature Ability", Snow Cloak, which allows it to become more evasive during Hail, and thus both Gen IV Eeveelutions can be said to specialize in weather-based Abilities. Unlike Leafeon and leaf Guard, Glaceon wasn't the only Pokemon upon its introducing to have the move as its primary Ability as another Generation IV Cross-Generation Evolution had it as well, but I still think it can be considered a Signature Ability regardless.
At the end of the Battle Frontier season , May got an Eevee leading to much speculation about what Eeveelution she'd obtain, with the popular idea being it was hinting at a new Eeveelution soon to be revealed in the upcoming Diamond and Pearl---May left before Eevee evolved though, but a few years later she came back during Diamond and Pearl with a Glaceon, so I assume that the anime writers definitely were aware of new Eeveelutions and set that up ahead of time. It's not too surprising, since they did it with another Pokemon in Battle Frontier who also received a Generation IV evolution. Anyway, May's Glaceon battled the new girl, Dawn, in the finals of the "Wallace Cup" Contest, and lost to an unevolved Starter Pokemon. Yeah, once the new girl shows up, the old one is pretty much irrelevant. Glaceon's only other singular appearance of note was when it quickly popped up in Best Wishes under the ownership of QUEEN CYNTHIA, the Rosalina of Pokemon. She never had one in the games as far as I recall, and only appeared in one scene so it wasn't much. But still, if we could only all be so lucky to be owned by QUEEN CYNTHIA.