Mares just wanna have fun...

And geldings and stallions and colts and fillys too, of course.

http://www.horseballs.com/user/video-intro-qt.html

Some horses really just seem to go nuts and love those toys. I've seen them do similar things with orange traffic cones they've gotten hold of, too.

EDIT: Don't be put off by the company name in the url. It's totally worksafe.