Edit in JSFiddle

var items = $$('li');

items.each(function(item){
    var b = new Element('b').inject(item);
    b.set('morph',{
        duration: 'short',
        transition: 'back:out'
    });
    item.set('tween',{
        duration: 'short'
    });
    item.addEvents({
        mouseenter: function(){
            b.morph({
                'left': 8,
                'border-left-color': '#1b60a6'
            });
            item.tween('color','#1b60a6');
        },
        mouseleave: function(){
            b.morph({
                'left': 0,
                'border-left-color': '#000'
            });
            item.tween('color','#000');
        }
    });
});

li {
    position: relative;
    padding-left: 28px;
    font-size: 30px;
    cursor: pointer;
}

li b {
    position: absolute;
    top: 6px;
    left:0;
    height:0px;
    width: 0px;
    border-style: solid;
    border-color: transparent transparent transparent black;
    border-width: 12px;
}