Hello, this is Dr. Linda. I hope I can help with your question today.
I am very sorry for your concerns about your horse. It is a scary situation when there is a sudden onset of unexplained symptoms.
There are several observations that come to mind with the symptoms you describe. One would be some injury or trauma that occurred during the day that resulted in some type of cervical (neck ) instability to a pelvic fracture.
The next thing I would rule out would be neurologic disease, caused by a virus ( ie West Nile or one of the Encephalitis serovars ) . This could be ruled out depending on what you routinely vaccinate your horse for.
Lastly, & least likely, some type of stroke, or even tumor in the brain causing his instability.
Please consider having your vet out to evaluate your horse. He/she may be able to diagnose on the spot, do some diagnostics to try & narrow down the cause of his symptoms, & hopefully start him on a course of treatment to make him more comfortable & possibly return him to his normal health.
At 29, he is definitely a senior citizen in the horse world, & it may be his time. But I would have him examined before you make any final decisions or just chalk it up to old age.
I hope this makes sense & gives you the information you were looking for. Please let me know if you have any further questions or concerns.
Thank you & all the best for a speedy recovery for your horse.