The 4 legged V-8 horse

That's a fairly extreme case but a Google search has images of similar and even more advanced cases. It's called equine lordosis, and surprisingly 'the affected horse functions as though he or she were normally conformed...the spinal deviation does not have a disabling effect," Even the most severely affected individuals can be trained and ridden and can participate in horse shows' - See more at: http://equusmagazine.com/article/swaybacks_081205-8221#sthash.FFImGtYV.dpuf
It's caused by the muscles and tissues of the back failing to keep the spine in tension, but they still support the spine, so it's not like the back is broken. Sometimes caused by extreme age, some claim it can be caused by riding too soon or overloading a young horse but it seems to be mostly genetic, or at least some horses are just more genetically inclined to get it.

I know, it looks like it must hurt like horse hell. There are a few other conditions and injuries that mimic the condition but there is severe discomfort and pain involved and are not easily confused with lordosis.

"Pardon me while I have a strange interlude"- Marx