@keyframes roll {
  0% { transform: translateX(-200px); }
  50% { transform: translateX(200px); }
  100% { transform: translateX(-200px); }
}

.ball {
  display: inline-block;
  width: 40px;
  height: 40px;
  background: red;
  border-radius: 20px;
  animation: roll 3s infinite;
  animation-fill-mode: forwards;
}