Edit in JSFiddle

$('#container').highcharts({
  chart: {
    events: {
      load: function() {
        this.xAxis[0].paddedTicks[this.xAxis[0].paddedTicks.length - 1] = "butterfly";
        //console.log(this.xAxis[0])
        this.update({
          xAxis: {
            categories: this.xAxis[0].paddedTicks,
            max: this.xAxis[0].tickPositions.length-1
          }
        })
      }
    }
  },
  xAxis: {
    type: 'category',
    max: 100
  },

  series: [{
    data: [0, 1, 56, 32]
  }]

});

<script src="http://github.highcharts.com/highcharts.js"></script>
<script src="http://github.highcharts.com/modules/exporting.js"></script>

<div id="container" style="min-width: 300px; height: 300px; margin: 1em"></div>