Roxy fell in the old tank at about 4:30 and was stuck for about 30 minutes with only her head visible above ground.
About a dozen firefighters managed to pull the horse out of the hole using a pulley system and help her stand up again.
Firefighters said the horse was visibly shaken and suffered a cut on her front leg. They bandaged the leg before a veterinarian arrived.
After getting hosed down and cleaned up, Roxy was able to walk back to the barn under her own power.
Roxy's owner, Martha Johnston, said she's confident her horse will make a full recovery.
"Roxy has helped to teach many children over the years how to ride," Johnston told firefighters. "She's a sweet old horse that we love dearly. The firefighters were great this afternoon."