Jump jockey Paul O'Neill is being investigated after TV coverage showed him apparently headbutting his mount before a race at Stratford.
O'Neill was thrown from the saddle by City Affair
The Horseracing Regulatory Authority (HRA) will hold an inquiry after examining footage of the incident.
A spokesman for the RSPCA said O'Neill's behaviour at Sunday's meeting was "totally stupid and irresponsible."
The 26-year-old jockey was wearing his riding helmet when the incident took place before the race at 1510 BST.
HRA spokesman Owen Byrne said a disciplinary hearing would be held.
"We haven't got a date set but we are going to go for some time next week - our inquiry day is usually on the Thursday," he said.
City Affair, trained by John O'Shea, was unruly before the start of a novices' selling hurdle and unseated O'Neill.
The jockey got to his feet and grabbed the reins, pulling his mount to him, before lowering the butt of his helmet towards the horse.
The pair eventually went on to finish fourth in the race.
"We trust the racing authority will deal appropriately with the jockey," the RSPCA spokesman told BBC Sport.
"We are satisfied the horse wasn't actually distressed or suffered any injury as a result of this incident, but it sets a very poor example and is not the way to behave with animals.
"People in a position of responsibility, dealing with horses, shouldn't be behaving that way."
The incident was shown by TV channel At The Races, but racecourse stewards did not have video evidence at the time.
The O'Neill incident is highly unusual, although there have been cases before of jockeys hitting out at horses.
Timmy Murphy was given a seven-day ban in 2004, reduced to just one day on appeal, after he struck out at his mount Semi Precious following the pair's fall at Plumpton.
On that occasion, the Irishman seemed to throw his whip at the horse in frustration as it got to its feet and galloped away.