She started to "kill" the ball, common dog play. I caught a glimpse of something shiny on the ball and was worried it was a tack or a nail. As I got closer, I realized that my previous double quotes needed to be edited out.
She realized I saw and decided to let me in on her secret. She apparently killed it before it could camouflage to an actual tennis ball again. Here's a closeup:
Here are some previous kills I found once I knew what to look for. The last two pictures, I think, indicate a scalped mutant and in a separate location its oddly crusted brain.