Prepare yourselves, this is going to be a huge list!

Why are you not Fire/Psychic? Ninetales learns several Psychic moves and has supernatural powers.

You need Psychic as well. I mean, all Pokédex entries describe how Golduck has psychic powers, and he can learn a handfull of Psychic moves, yet it isn't Water/Psychic.

Feraligatr is evil and an alligator, enough reasons for it to be Dark right?

Again, Noctowl has Psychic powers as well, and can learn many Psychic moves.

Ledian has FISTS, learns several Fighting type moves, and even gets Iron Fist. I actually understand it isn't Fighting type, since its Attack stat is soo pathetically low.

You sir, are a tree. Y U NO GRASS?!?

Granbull needs Dark type. It is intimidating as hell, and gets Crunch and Bite, so...

What's with all the Psychic types? The afore mentioned reasons to Golduck, Noctowl and Ninetales apply to Stantler as well.

Water/Flying. OK, I get it. It can't fly, nor it has any wings. But it is the bringer of the northern wind! Suicune is more related to wind than it is to Water.

You. Lugia lives UNDERWATER, is the guardian of the SEA, can create WHIRLPOOLS, learns WATER type moves, can be found in WATER or near WATER in every game it is available in, and it is a Psychic/Flying type? Geez Game Freak, get your sh*t together.

Surskit; Bug/Water. Evolves; Bug/Flying. I get it gets wings, but that type combination is so cool!

It should be Bug/Electric, I don't know why.

It is literally made out of Rock, so it should be Ice/Rock.

Counterpart to Gorebyss. Huntail gets Dark type moves, Goreyss gets Psychic type moves. Huntail lives deep underwater, where it is DARK.

Like I said above, it should be Water/Psychic

Serperior should have been the first Grass/Dragon. I first thought it was Dragon actually.

It has huge fists and is super agressive. It should be Fire/Fighting.

When I first saw it, I thought it was Ice/Ghost. Therefore, it should be said type.

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I thought it was something like Ice or Fighting type. Not Ground/Ghost. Make it Ice/Ghost or Fighting/Ghost.

Fighting/Flying, because 'MERICA.

and its evolutionairy line should be Fairy/Grass. I mean, it learns several Grass moves, it holds or is a flower, lives in Grass and has Flower Veil as ability.

And its evolutions should have been Dragon/Water. Probably too OP, but it at least makes sense.

I first thought it was like Ground/Poison, but nope, another Dragon legendary. I hope its future forms change its typing to Dark/Ground and Fairy/Ground.
Z not far...