Dosidicus Giygas asks:

There’s an interesting parallel in Gen I between Eevee’s three original evolutions and the three Legendary Birds in terms of typing. Fire, ice, and lightning are common elemental distinctions in RPGs with magic/energy/psionics/whathaveyou, so it makes sense that Pokemon would draw from this tradition for inspiration, though it’s a little odd that there is a discrepancy between Vaporeon (Water Type) and Articuno (Ice Type). Any thoughts on why that is? Furthermore, why didn’t Game Freak apply this logic to the starters, who are halfway there anyway? For something more varied/interesting? For a better justification of type balance?

Type balance isn’t exactly right, because I don’t think it’s about fairness, or at least not entirely, but it’s something like that.  Grass/Fire/Water has this nice rock/paper/scissors relationship that serves as an easy and intuitive introduction to one of Pokémon’s core mechanics, which is a pretty valuable thing for new players.  It doesn’t really work if you try to shoehorn Electric in there, because thematically there just isn’t an obvious relationship between Electric and Fire.  Other games that use Fire/Ice/Lightning don’t usually have “type advantages” in the same way as Pokémon does; several iterations of Final Fantasy, for example, have Fire and Ice being strong against each other, with Lightning doing its own thing (often being strong against mechanical enemies); Final Fantasy X adds Water as a fourth element to form another opposed pair with Lightning.  Pokémon just has different needs to those games.

As for the substitution of Water and Ice in the Eevee and legendary bird trios… well, ice is water, so thematically it’s almost not even a discrepancy at all.  Almost all Water Pokémon learn Ice attacks, and I think it’s also worth remembering that in generation I there were only five Ice Pokémon, none of them single-typed, and three of them were Water/Ice.  Furthermore, giving Eevee an Ice-type evolution instead of a Water-type one would have messed up the neat Fire Stone/Water Stone/Thunder Stone evolutionary scheme (because, again, we’re in generation I, so there is no Ice Stone and Eevee only has three evolved forms).  You could add an Ice Stone to the game – but there are so few other Ice Pokémon who could use it.

Really though, the weird thing is that Water is even a type at all.  It’s fairly unusual in fantasy game systems for Water to be a damage type, because… what does that even mean?  Water isn’t inherently harmful, not like fire or electricity, anyway.  With, again, the notable exception of some iterations of Final Fantasy, most systems either assimilate water to ice/cold, or have water-based attacks do physical damage – which makes sense, because when you’re struck by an attack like Water Gun or Hydro Pump, you’re being injured by the crushing force of the pressurised water.  Pokémon has to have Water as a damage type, though, because – and we should probably recognise that this is actually a bit unusual – all its damage types are also creature identity types, and vice versa.  And Pokémon wants a Water type for all its aquatic creatures.  This is actually something I’ve talked about changing in the past; if I were completely rebuilding Pokémon from the ground up, one thing I’d like to do is drastically reduce the number of types.  One of the ones on the chopping block would probably be Water – to be replaced by “amphibious” and “aquatic” as movement classes or terrain affinities or combat modes or something, rather than types as we know them.