You can for example use an already animated looped gif of an animal walking. Then you can just change the position of the gif with JS.
var elem = document.getElementById("animal");
var pos = 0;
var id = setInterval(frame, 75);
function frame() {
if (pos >= 100) {
pos = -10;
} else {
pos=pos+0.25;
elem.style.left = pos + '%';
}
}
solved Animate animals running accross the screen in Javascript [closed]