Charlotte McPherson, from Kidderminster, in Worcestershire, was reported to the RSPCA after she was photographed riding Thoroughbred horse named Troy at a public event in March.
However when official pictures of the event were posted to Facebook, she was reported to animal charities.
Earlier this week she admitted two counts of animal cruelty at Birmingham Magistrates' Court.
The 22-year-old was charged with causing unnecessary suffering by failing to investigate and treat the cause of his poor body condition, and failing to protect him from pain, suffering, injury and disease by riding him in this state.
She has now been prohibited from keeping horses for 10 years. She was also given a 12-month community order, 160 hours unpaid work and must pay £300 costs and a £60 victim surcharge.
Following an RSPCA intervention, Thor was treated by the Retraining of Racehorses charity and has now been re-homed.
RSPCA inspector Suzi Smith said: 'Thor looked incredibly thin and you could see pretty much every bone in his body. He was obviously unwell, and after a vet examined his body, his body condition was rated zero out of five because he was that thin.
'McPherson continued to ride him twice a week in this condition, including at a fun ride in Bissell Wood, Blakedown, in March.
'Not only was he thin, but he had a sore on his spine which was directly underneath the saddle. Thor would have been in a lot of pain while he was being ridden.'
She added: 'Thor has done amazingly well and he has now been rehomed. He is loving his new life and when he trots across a field, you would find it hard to believe what he was like earlier this year.'
Neville Cope, 74, who organised the fun ride, said: 'I'm over the moon that she has now been seriously penalised because I was appalled by it. She hid the rib cage of the horse by putting a blanket over it.
'We have a vet who goes around especially to make sure they're all OK but the [blanket] on the horse meant they couldn't see [how horribly skinny he was]. When she got back from the ride and took the [blanket] off we were all so appalled by it.
'I'm delighted by the justice which has been dealt and quite right too. I've been involved in horses all my life and never ever have I seen a horse in as bad a state as this, I was absolutely disgusted. We have a vet to make sure all the horses were alright but unfortunately we missed it until after.'
(Daily Mail - Dec 7, 2016)