[Discord, Chrysalis, and Sombra come to mind...]

They weren't necessarily aiming for her. They decided it would be hilarious to bake a bunch of poisoned cookies and distribute them among the people here. Fluttershy was one of the ones who had the bad luck to receive some.

Not that that makes it any better, mind you. [Arguably worse, since it shows they didn't care who they hurt, they just wanted to hurt someone.]