Today, balls for the popular sport of Goal Roll are made of leather or occasionally Grumpig hide. Not too long ago, live pokemon were used instead. There are many pokemon which are capable of filling this role – most pokemon who can learn the attack Rollout can do so, although some like the rock-type Geodude are too dense to kick around. Voltorb and Electrode are excellent for the role, being perfectly round, but far and away the most popular are Sandshrew.
A Sandshrew's brick-like texture is both recognizable and easy to kick or punch, the grooves making it fairly aerodynamic. Their tough skin and high defenses means that these moves will not especially hurt the pokemon: indeed, the goalies often find themselves more injured by Sandshrew kicked at a high speed. Furthermore, although hardly as ubitquous as Rattata, they are a fairly common pokemon who can be obtained for ballgames.
The Sandshrew themselves (or occasionally other ball-shaped pokemon) participated in the matches back then: each team brought its own ball. Although it was a penalty shot if they left their round shape for more than two seconds, they could still attack somewhat and direct their flight to influence the match in their team's favor.
In recent years, superior balls have been found, and the Society for the Prevention of Cruelty to Pokemon has managed to ban live pokemon from the professional leagues, where Jigglypuff and Voltorb were also occasionally used. This move has been derided by Goal Roll purists, who deride the new balls as less aerodynamic and unable to assist their teammates, blame them for a decrease in offense in the modern game, and long for the end of the dead ball era.