Looking at movesets again today and another slightly odd move has me contemplating it: Energy Ball.
Gen III and IV were typically where a lot of older moves which weren't always explained in great detail got slightly more illustrative descriptions (Agility, for instance) thanks to their larger textboxes. Because of this, a lot of moves introduced in those gens haven't changed a lot in terms of how they're described - Energy Ball is no exception to this.
Gen IV description: The user draws power from nature and fires it at the target. It may also lower the target's Sp. Def.
PLZA description: The user attacks by drawing power from nature and firing it at targets. This may also lower targets' Sp. Def stats.
So that's a pretty standard Grass-type technique. As you'd expect, everything that learns Energy Ball by level is a Grass-type, apart from Flabebe (obvious, basically an honorary Grass-type) and... Swirlix, for whatever reason.
But since it's been a teachable move in every generation, lots of other Pokemon learn it too. Which fits for a relatively bland description like "the user draws power from nature": many different Pokemon should be capable of that. A huge amount of Psychic-types get it, as do a lot of Bugs, quite a few Fairies, and interestingly quite a few Ghosts (Gastly, Frillish, Yamask, Misdreavus, Sandygast). So do a handful of other Pokemon who live in natural environments or have a stronger-than-usual connection to the elements: Vulpix*, Stantler, Passimian, Castform, Glimmora, Terapagos.
There's a few species for which I question the logic of them learning (Magearna, Deoxys, Giratina, Gengar, Beheeyem, Xurkitree, Celesteela, Iron Moth) but I'm more interested in how many things don't learn it, especially Water-types. Octillery does, but that seems more to do with it being a projectile move (of which Octillery learns basically every single one) than because of a connection to nature; Manaphy does too, but that might be the whole "it's a legendary/mythical so it learns loads of moves" thing I mentioned in the last post; Frillish also does, but equally it's part Ghost so maybe it's just because of that.
But if any type represents nature outside of Grass and Fairy, it's surely Water: and even though Grass moves are a stretch for a lot of Water-types (given that many of them are fairly mundane aquatic creatures without much in the way of vast intelligence or elemental power) there's at least a few I'd think should be capable of learning it: Starmie, Lanturn, Politoed, Clawitzer, Lapras, Corsola, Araquanid, Tapu Fini, Dracovish, Walking Wake. Starmie is quite possibly the oddest exception given that it's a Psychic-type with mysterious elemental abilities.
Non-Water-types I'm also a little surprised have never been able to learn it:
-Zygarde (it's literally all about nature)
-Tapu Koko (all the Tapus are about nature - I can sort of see the thinking behind only Lele and Bulu getting it, but I don't think it'd be out of place for the other two)
-Landorus (it's not like it really needs it, but, again, is a nature-oriented species)
-Espeon (a notable Psychic exception; it learns Grass Knot, Trailblaze, and Morning Sun, but not this)
-Slaking, Furret, Miltank, Kecleon (all grassland-dwelling Normal-types with wide elemental learnsets)
-Ariados, Ledian, Vespiquen, Yanmega (all notable Bug exceptions)
-Altaria and Goodra (perhaps the most nature-attuned of all the dragons)
-Togekiss (learns just about everything else)
-Audino (as does this)
-Carbink/Diancie (the latter is especially odd, as most of the other pixie-like mythicals do - and Diancie is a very "natural" species)
-Hatterene (Fairy AND Psychic, and yet doesn't learn this despite learning moves like Life Dew, Magical Leaf, and both its native terrain moves)
*though that one could just as easily be due to its affinity to Psychic- and Ghost-types than its connection to nature