Whip, like Matt says, you just roll it across the floor and the ovals being offset causes the captured ball to roll from side to side as the toy rolls. As the baby gets older they'll push it and crawl after it again, and again, and again.

Ryan, I've got a template for a wood rattle for the little ones. It's a piece of hardwood, split with a portion routed out to form a hollow, then glued back together with a spoonful of rice inside. All smoothed with a cutout for easy holding, and of course vegetable oil finish. If they happen to break it open the rice is harmless.

There may be others, and it's easy to copy old designs that were common 40 to 50 years past, but no longer seen.