Okay, so like I am a big Rhyperior fan, but for the longest time I really hated him because of this misconception springing from his Generation 4 backsprite.
As you can see, he has his arm up, and because of the camera angle, it is behind his head...but I guess I did not look too closely on the DS, because to me it always seemed like the arm was coming out of his head. (And it wasn't too hard to think that the hand was just off-screen) Which, looking back...is just...weird...because I knew what his head looked like from the front sprite.
So basically, I assumed that it was his horn and it was just bigger and different when looking from the backsprite or something. That, or it was just some weird horned growth coming out of his head.
...I don't think I really discovered that it was his arm until I played Soul Silver around July-August 2012, because HGSS had the backsprite animations when you entered battle, and I saw the arm and head animate as two separate things. I also noticed that a bit of his elbow can be seen at the end, and I saw how the arm and elbow were attached.
So that sort of got rid of my "head tumor" Rhyperior impression and I started to like using him xD But...yeah...that was just...weird...