Investigators say the horse, which was severely emaciated, was found off of 11th Street and Highway 359 on Friday.
The SPCA said a $5,000 reward was donated by a fellow horse lover for anyone with information.
Equine veterinarian Dr. Virgina DeChant said the 16-year-old mare is tough and showing a strong will to live.
"Considering the horrible condition she is in, and how much she has apparently suffered, she is one tough girl," said DeChant.
In addition to starvation, DeChant said the horse's coat is matted with manure and the lack of farrier care has been causing pain to her feet and tendons.
The rescue organization has already introduced a slow re-feeding plan, and will provide a therapeutic hoof treatment during her rehabilitation.
The Brookshire Police Department and the Houston SPCA are asking anyone with information to please email the Houston SPCA at info@houstonspca.org or call Brookshire Police Department at 281-375-5000.