It's not really a Pokemon, but that ghost thing in Red/Blue & FR/LG. The one you need the Silph Goggles (or whatever they're called) to see, and it ends up being a dead Marowak. Yeesh, even my Pokemon were scared stiff! That thing gives me the Heebie Jeebies!
Also, the sprite of Golbat from R/B...Look at this thing;
*pokemonelite2000 image removed*
With his tounge just flappin' wildly about...Eww! *crumples picure of Golbat, burns and buries the ashes*