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.
Miss Harris described Polly as "fidgety" and said the thoroughbred mare had been nipping at her and others.
She added: "She then went to canter and I held her up because I didn't ask her to. She was throwing her head around and bucked, then I came out of the saddle and went over the horse's head."
Her barrister, David Westcott QC, said she should never have been allowed into Polly's saddle as she had only ever ridden ponies.
He also said the mare was "bred for racing" and had not been well schooled.
Mrs Miller said the teenager fell as she was riding Polly at a walking pace down a hill in the field.
She also said she spoke to Miss Harris's mother before letting her ride Polly and believed Ashleigh was a more experienced rider.
Winston Hunter QC, for Mrs Miller, said: "Ashleigh gave Keiran an impression that she was indeed a competent rider.
"There's no evidence Mrs Miller was aware that she had only ridden ponies."
Judge Graham Wood QC will give his ruling on the claim at a later date.
A teenager who was paralysed after falling off a horse is suing her ex-boyfriend's mother for £3m.