@ Melody, take Dudduo the Doduo. One of Dudduo's heads is a dud. It just kind of hangs there, all limp like. As such, it can't use any attacks that would require it to use its other head or has something to do with the number 2 or 3 (Since this is gen 1 and the attacks are so few, I'll list them here: Fury Attack, Tri Attack, Double-Edge, Double Team, Mimic [just roll with my reasoning here]). It also renders him a little slow mentally, so he/she always growls uselessly before attacking. For some strange reason, it seems attracted to the Celadon Gym's Leader, Erika. In order to evolve, it must solo her entire team, after which the reason is revealed! The strange chemicals in her Pokemon allow him to grow another head, evolving into Dodrio. However, one of it's heads is still a dud. As such, you must rename it to "Dudrio" at this point, and while most of the old restrictions no longer apply it still cannot use Tri-attack.