[Naix]

Someoneelse, take Scorpio the Gligar. Throughout its lifetime, Scorpio believed that it was actually a Bug/Flying-type, rather than Ground/Flying, since it's obviously a gliding scorpion. This causes Scorpio to have... unusual tendencies as far as behavior and battling style go. For example, Scorpio believes that it should always know at least 1 Bug-type attack so it can always have STAB. Also, as the first line of defense scorpions use in real life is to use the stingers on the end of their tails, Scorpio must always know Poison Sting and must use it at the beginning of every wild Pokemon encounter, even if the enemy's a Steel-type.

However, soon after you catch Scorpio, it hears from your other Pokemon that it is actually part Ground rather than part Bug. This causes Scorpio to distrust your other Pokemon to the point where it refuses to partake in any double battles with the rest of your team. (Note: This does not prevent Scorpio from participating in double battles with other trainers, like Cheryl or Barry. Also, the above restriction can be ignored if you have no choice but to send Scorpio in.) It also makes Scorpio decide to lead your party through Pastoria Gym (without fainting once) to prove its case. After this is done, Scorpio may evolve.
Have fun!

Also, Dragoon124, Misdreavus are unavailable in Platinum, and Someoneelse said he can't trade, so you'll probably have to change your challange.