What are the most disturbing nursery rhymes you know?

I heard one once that I have never been able to find again. It's the original version of "All the pretty little ponies," and has a part where it talks about a pony with "flies picking out his eyes and he's crying for his mammy." It's supposed to be about slavery in the united states, I believe... but I've never been able to find it since I first heard it.