Ashleigh Harris was 14 when she broke her back after falling from Polly Perks, owned by Rachel Miller. She now has no function in her legs.
She was riding Polly in a field in Mathern, near Chepstow, Monmouthshire, when she was injured in 2012.
Miss Harris, 18, of Lydney, Gloucestershire, is suing Mrs Miller at London's High Court.
She claims Polly "misbehaved" and broke into a canter of her own accord, throwing her off in the process.
But Mrs Miller says the horse was blameless and Miss Harris fell after losing her balance as she descended a "short and gentle slope".
The court heard Ashleigh had been taken to a field on 22 September 2012 by Mrs Miller, the mother of her then boyfriend, Keiran Miller... READ MORE