Fairly early in the game, it was possible to catch Misdreavus, and the Dusk Stone was fairly easy to get in that game, as well. So I caught a Misdreavus and named her Missy and as soon as I got the Dusk Stone and got Misdreavus to a level where she wouldn't learn anything more useful as a Misdreavus, I evolved her into this new, witchy form of hers.
I wouldn't say that I like Mismagius' design more than Misdreavus. In fact, I like the un-evolved Misdreavus better. The only reason I like Mismagius is because she is a more powerful version, and I like more power. While I love cute Pokemon, I want my Pokemon to be strong, and any way to make them stronger, I take. To be fair, if Pokemon battles were not the basis of the game, I probably would've just forever kept her a Misdreavus forever! But Pokemon is all about having strong Pokemon, so I relented and went with the stronger version, even if it isn't nearly as cute.
Photo credit: bulbapedia.bulbagarden.net/wiki/Mismagius