A friend swears that as he walked past one, a horse growled at him!

I've never known this, could he be having me on?

it depends what you class a growl as, but i've definitely heard a horse make vocalisations similar to a "growl" sound before

No they dont growl but they do make a low rumble type noise which usually means they want something.

Thanks

They nicker, which is a noise that sounds a bit like a growl. It's a noise that they make when they see their food bucket or their owner.