Cows have white/green boxes on them

Hi. I was playing on my single player survival world when I came across cows with these box-like things inside of them. At a distance, they appeared white. As I came closer, they turned lime green. I don't know if this is a texture error, but when I tried re-logging, it was still there. This is in the 13w16a snapshot of Minecraft. Thank you.

Username: XxCrazyMattxX