Alvin
One item sold under the name "Walker Ball"
is for a walker (thing designed to help injured or disabled people to walk)
which only has wheels in the front and has legs (like chair legs) in the back.
This "Walker Ball" looks like a tennis ball with a partial hole in it.
You put them on the back legs of the walker and it allows the
person using the walker to be able to slide the walker on the ground easier.
You could also put 4 of them on a walker which only had legs and
no wheels to help it slide.
Sorry this website in the comments doesn't have many pictures, but I can see
if I can more on the web . That is , if this is the kind of "Walker Ball"
you are talking about.
You could also call a squeezable ball used for
wrist exercise a "Walker Ball" if it was sold to
be used while you were walking.
Robert S
It's a ball you walk like a dog.