The condition you are concerned about is called dsld or degenerative suspensory ligaments.
I've never seen a horse recover once the fetlock drops with the exception of foals.
Mr. Prospector had the condition in later life. A foal of his developed it as a 2 yr old and was put down from it. I suspect the problem is much more common than most people hear about.
My thoroughbred has it. She has had lameness issues for most of her life, and it wasn't until her ankles dropped, that I realized what was wrong. First it was one ankle, than a few months later the other dropped.
I doubt your horse will return to racing sound even if it is not dsld. If the ligaments have weakened so much from the spider bite, they probably won't go back to normal.
If you want to use the horse for breeding purposes, I would wait a year to ensure it isn't dsld.
Do you have pictures?
Dsld is not uncommon. I know 1 paso with it, one qh, and 2 saddlebreds. http://www.drf.com/blogs/fond-farewell-two-punch
I know there are some better pictures of Mr. Prosecutor's back legs but I can't seem to find them.