Ahhh, it was a few years ago... A bunch of them swarmed that actually did that. A bunch of people got their souls stolen, we had to use special balls to catch them and send them to the professors to get them back, it was a pain... I'm still convinced they were the product of some kind of bothersome experiment, though. Haven't heard anything about them since.

Natural ones you'll find after Nincadas evolve are fine, though.