It took firefighters from Duns, Galashiels and Eyemouth almost 90 minutes to rescue Guinness, a grey, weighing in at 600kg.
The horse, belonging to Rebecca Morgan, had walked over the six inch thick concrete slab many times before, but unfortunately on this occasion it gave way under the animal's weight.
Laura Shepherd, a vet from Renton Swan and Partners, attended the scene almost immediately and sedated the horse while firefighters worked out how to lift him out of the tank.
They made a harness out of a hose and used a farmer's telescopic fork lift to rescue the horse.
Guinness suffered a couple of cuts and grazes, but was otherwise unhurt.
Speaking to 'The Berwickshire News' on Friday, Rebecca's mother, Margaret Morgan, said everyone at Sunnyside is just glad that Guinness is on the mend and escaped only with minor injuries.
"He's quite a lot better now and although he's a bit battered and bruised he's a lot brighter. We've had excellent veterinary care for him and he's been well looked after.
"Once we'd got him out of the hole we washed as much of the sewage off as we could. This proved to be quite hard as after being sedated he was quite unsteady on his feet."
The Morgans were first alerted to the incident when a French student who is currently working for them heard a loud crash and saw the horse going into the tank. Margaret said it was a good job that they were alerted straight away.
"We were with Guinness in around 10 minutes after being told what had happened. The tank is around eight feet long and that's where the problem arose - Guinness is only 6' 2" so he couldn't drag himself out. He did try and managed to get his front feet out but couldn't push himself up.
"It was just a great relief for us all when he was finally taken out and without any broken limbs.
"I think the accident came as a big shock to everyone and now the tank is fully fenced off."
Fourteen-year-old Guinness has been at Sunnyside for six years now and is used in both riding and vaulting for the disabled schemes. He also has four years experience on the international vaulting stage.