What is the meaning of this turnip?
The turnips can be destroyed for 1000 points. They aren't evil or special. You'll find some others littered across other levels, but they don't provide any special functionality currently. You do want to keep an eye out for golden eggs though.
In the level above (Pig Bang 13), you can destory this one with the blue bird, but I'm pretty sure to 3 star the level, you should be finishing the level with the first bird.