Dear god... Exeggutor... My mother used to tell me how she had dreams about it. Like this one time she told me about having a dream where she went fishing with her team of Pokemon from Pokemon Stadium, and poor Exeggutor felt left out. So she made an arm vest for it with a pole attached to the hands... It's freaked me out ever since she told me that. xD
Also, Ditto. Remember the anime? Most Ditto can fully transform, facial features and all. What if a Ditto transformed into a friend of yours, waited until you weren't looking, transformed into a Charizard and flamethrower'd you, then transformed into you and lived out your life? That's even worse than Ghastly possession. o_o
(Too much coffee, I guess, but seriously... You know, that might make a nice fan fic... Oh, and a twist! Someone catches the 'Ditto you' in a pokeball by accident and starts freaking out! xD)