The Evening Standard reports that former Sun and News of the World editor Brooks was allowed to keep the retired horse for over a year.
The loan was made in 2008, the year after former News of the World royal editor Clive Goodman and private investigator Glenn Mulcaire were jailed for illegal interception of voicemails.
It was also offered while Lord Blair was the Metropolitan Police commissioner, although he claims he was not aware of the situation. The horse was re-housed with a police officer in Norfolk in 2010.
Speaking to the newspaper, a friend of Brooks and her horse trainer husband Charlie said: "Rebekah acted as a foster carer for the horse. Anybody can agree to do this with the Met if they have the land and facilities to pay for its upkeep."
Brooks's spokesman added: "It's well known by people in the horse world that the Met looks for homes for horses once they retire. Rebekah took on a horse and effectively acted as a foster parent for it for a year or so.
"The Met horse team comes out to make sure your facilities are right and proper. It's just a way of giving a temporary home to a horse that has had a distinguished service in the Met. It went off to a retirement paddock in Norfolk once it couldn't be ridden anymore."
The Met Police said that it was routine procedure for retired police horses to be loaned to members of the public after their working lives, but the arrangement with Brooks is likely to raise fresh questions over the closeness of her relationship with officers.
A Scotland Yard spokeswoman said: "When a police horse reaches the end of its working life, Mounted Branch officers find it a suitable retirement home. Whilst responsibility for feeding the animal and paying vet bills passes to the person entrusted to its care at its new home, the horse remains the property of the Metropolitan Police Service.
"Retired police horses are not sold on and can be returned to the care of the MPS at any time. In 2008 a retired MPS horse was loaned to Rebekah Brooks. The horse was subsequently re-housed with a police officer in 2010."
The revelation comes a day after the Leveson inquiry into press ethics and standards heard that the relationship between News International and the Met was "at best inappropriately close and at worst corrupt".
It was revealed that Brooks was briefed by a senior Met officer on the progress of the original inquiry into phone hacking, and even asked how far the investigation should go within the News of the World.
> Rebekah Brooks quits News International after hacking scandal
> Former NOTW editor Rebekah Brooks welcomes baby girl