Hmm… tough question. I don't know; in order for a Pokémon to be considered pure flying, maybe it had to be completely made of air, which they probably couldn't think of an idea for? d: The only pure flying-type Pokémon we ever had is Tornadus. It sort of fits the requisite, but not
exactly, though, as the top part of its body has a human/animal-like form.
I remember when everyone thought we'd never get a pure-Flying Pokemon but we did eventually get one in Tornadus as you mentioned, if you don't count Sky Plate Arceus. And then again, most Flying-types we've had so far were based on birds and other creatures with wings. Some think a Pokemon has to look like something that would represent its type best, but then again we always get the odd variety every now and then of a Pokemon with a type that doesn't really go with its appearance, like Inkay and Malamar. They are Dark/Psychic-types yet don't really look the part, being a squid and all. Their appearance at first glance would scream Water, and don't forget Dunsparce too. The thing almost looks like it'd be a Ground-type imo but it's a Normal-type. And not every Water-type is based on a sea creature or some sort of animal that spends life in water.
But that's like saying all Grass pokémon should be entirely plant-based. Which a lot of them clearly aren't. Like Treecko, Chesnaught etc.
Yeah, Treecko and Chesnaught are some of the biggest examples of Grass-types that lack a plant-based feature but then again most Grass-types are green in color or have a plant-based feature so it's usually easy to spot them. And we also have some Fire-types that don't have visible flames on them, like Tepig, Pignite, Torchic, Combusken, Vulpix, Ninetales, and even Fennekin's entire evolution line, however Braixen and Delphox use twigs like wands that light up with flames when they perform attacks.