I was in my friend's pasture a while back and saw what looked like part of a hoof- just a part that broke off, I thought, right? Nope. I walk closer, and get a good look at it... a real good look. It was the entire hoof. It looks like where it fell off was at the coronary band, and the hoof wall was there, along with the whole frog. And to top it all off, it was fresh. It stunk. I ran back to check all the horses, mainly their hooves, and all was well.

 What would cause a hoof to fall off like that? I've searched hours on the internet, but there's nothing about hooves falling off. Anyone know the cause?

You checked all the horses and they were fine? Are you sure that's what you saw?  What you're describing sounds like what happens when a hoof slough's off.  It's not good and you would have a horse in your barn that was in serious pain and would not be able to walk.  I've never personally seen it happen, but have seen pictures and read about it.  It can happen for many reasons.  The one I read about was a horse that had gotten her foot caught in some wire and the blood supply was cut off to her foot causing her hoof to slough off.  They tried to save her but ultimately had to have her put down.

I think maybe you saw something else.

My friend has gotten rid of a few of their horses before I was out there, so if it was one of the others, I don't know. But the horses they still have are fine.

eeewww, poor horse.

why don't you ask your friend what happened to the horse whose hoof fell off.  i mean you can't miss noticing a horse who only has 3 feet.