How do I counter Onager sneak attack?

So I'm in a 4v4 match. I was placed at the far end. I HATE being placed as the end-guy because I'm very weak when it comes to people rushing me and I'd rather be a support for my team. That's another story.

Anyway!

So I hear some rumblings in the fog of war. I remember back in the 90s when I played that Onagers can cut through the forest. By the time I remembered this and tried building walls around where I thought the enemy was cutting through the forest. It was too late.

So how do you really stop this from happening? Do I need to build the great wall of China around my base or what?