What existing Pokemon do you think could be mistaken for a different type than they are?
The biggest factor would of course be the appearance itself of the Pokemon, but also other factors such as moves it can learn, Pokedex entries, Egg Groups and other things.
Some example that come to my mind are:
Golduck - Water/Psychic (It learns a lot of Psychic moves and I was convinced it was actually part-Psychic when I was younger.)
Flygon - Dragon/Bug (It's very bug-like in its appearance and it's in the Bug Egg Group.)
Shelmet - Bug/Steel (I was quite surprised when I realized that it wasn't part Steel type, especially since Karrablast gains that type when evolving upon receiving Shelmet's shell.)
Lugia - Water/Psychic or Water/Flying (I always thought of Lugia as a Water Type and kept forgetting whether it was Psychic or Flying. This might have been because of the second Pokemon movie where it dwelt under the sea and all the references to the sea in its 'dex entries.)
So, any other ideas? What Pokémon do think could be mistaken as belonging to different type? Or are there any Pokémon you're convinced should be another type than they are?