Is that Normal?

We have our two donkeys, Lilly and Sunny, in with our old horse, Daisy.  One day I looked out the window and it looked like Sunny had a long string hanging out of her mouth.  Turns out she was eating Daisy's tail.  We kept noticing Daisy's tail was getting shorter and choppier all the time.  Both Lilly and Sunny were doing it.  We couldn't believe Daisy was just letting them eat her tail!  I told Fred maybe they are planning on eating the whole horse, but just started at one end. 

I "googled" donkeys eating tails and apparently I'm not the first person with this problem.

Weird.