I had one about Hydreigon. Though, I don't think that is my mistake, but rather mistake on everyone that interprets Hydreigon's pictures and sprites though general aesthetic sense.
Everyone and every website I visited said Hydreigon's "legs" are shortened or atrophied or degenerated through the evolution process. For me personally, those might look like "legs" by first glance, but it is really not legs, but rather feels like protruding "fur" to me. One proof is that for every animating sprites you can find on games, and all Hydreigons introduced in anime, their "legs" never move.
That is something like Espeon's fur protruding beneath its ear, where I believe someone may had mistake Espeon had two pairs of ears before.
They're just vestigial legs. Some real animals have remnants of limbs that they can't move due to having evolved out of the need to use them.
On a note pertaining to the same Pokemon, I was met with some confusion when playing Gates To Infinity, as there, it's depicted as having large black and red eyes, as opposed to small red eyes surrounded by black skin like in the other media. I presume it was done for the purpose of making its expressions easier to see.