Put the summary of the bug you're having here

Horses attached to leads clip through fences

What I expected to happen was...:
Describe what you thought should happen here

Fence should have contained the horse

What actually happened was...:
Describe what happened here

The horse passed through the fence

Steps to Reproduce:
1. Build a fence surrounding a 2x1 block area
2. Spawn a horse into that area
3. Attach a lead to the horse
4. Walk away