I'm sorry for your loss. I was about to post that your description sound more neuro than blind and to have a vet out to asses for EPM asap. Even blind horses/ponies can sense something as large as a human and really try their best not to stumble into them. I'm not sure how many other horses you have or how experienced you are as an owner, but please keep an eye out for any other horses that may be in the general area or have eaten the same feed. EPM is spread by possums, in their feces, and I know many possums like to break into feed storage areas. If they defecate, then any horse exposed to that feed can exhibit signs at any point in time. Some horses are affected mildly and slowly. Others, acutely and very, very quickly. One of my favorite horses when I was younger was found down in his stall after a mild stumble in his workout the day before and was dead within 48 hours despite any and all efforts to keep him alive. I've also known horses who were neglected that probably displayed symptoms for months before treatment and aside for some hind end disconnection, are perfectly normal and healthy after treatment.