The Queen has named the most senior animal in the British Army - a Drum Horse called Perseus who has the rank of Major.
Towering over the Queen, the imposing animal, which is 17.1 hands high, was given his new name by the monarch when she visited the barracks of his unit, the Household Cavalry Mounted Regiment, with the Prince of Wales.
At one point Perseus - named after the character from Greek mythology - who was ridden by Lance Corporal Richard Brown in full ceremonial dress, appeared to bow to the Queen.
The Queen's in-depth knowledge of horses is well known in racing circles but even she was intrigued during the visit when she met a horse painted with a skeleton.
It is being used to teach Troopers from the Regiment's two squadrons - The Life Guards and The Blues and Royals - about the anatomy of the animals they ride at major state occasions like Trooping the Colour and the State Opening of Parliament.
Educational company Horses Inside Out staged the demonstration with anatomist Gillian Higgins from the organisation explaining her work to the Queen.
The monarch joked at the end when she asked "does that come off?" and was reassured by Ms Higgins the paint was "water-based, hypoallergenic and non-toxic".
Ms Higgins, of Horses Inside Out, said the Queen was intrigued to see the position of the animal's neck vertebrae, which are quite low down.
She added: "A lot of people are surprised where the neck bones are, they think it runs along the top of the neck. That's one of the reasons I started this organisation, there are lots of misconceptions."
The regiment's Commanding Officer, Lieutenant Colonel James Gaselee, asked the Queen to officially name the nine-year-old drum horse Perseus, which carries the drummer in the Household Cavalry Band.
The senior officer has a family link with Charles as his father, Nick Gaselee, a former National Hunt trainer, trained the Prince as an amateur jockey in the early 1980s, and his daughter, Sarah-Jane, was a bridesmaid at Charles and Diana's wedding.
The Queen heard how Household Cavalry drum horses carry the rank of Major and are senior to all other animals in the Army.
Perseus is currently undergoing intensive, specialised training for the role as the Blues and Royals Drum Horse, and is set to pass out on the Queen's Birthday Parade next June.
During the visit, the Queen met Troopers of the Blues and Royals and Life Guards with their horses, and was shown Johnnie, a young horse she gave to the Household Cavalry, which is undergoing training to be a charger for the Life Guards.
"She's quite small," the Queen remarked, as Lt Col Gaselee explained that the mare is only four years old, and so still slight for a horse, but will grow in muscle and build.
The Queen gave Johnnie a polo, and joked "horrid" as the horse bared its teeth and curled its lip.
In the stables, the monarch was also shown Wellesley, the Duke of Cambridge's grey charger which he rides at Trooping the Colour.
Meanwhile Charles visited the forge, at the Hyde Park Barracks, where he heard about the Farriers' Apprenticeship Programme from Farrier Major Chris Thomas.
He was then shown the tack room, where the Master Saddler, Corporal of Horse Samuel Belasco, outlined the painstaking process of preparing for major events such as Trooping the Colour, state visits and royal weddings.
Inside the tack room, two troopers were polishing their boots, helmets and other bits of their ceremonial kit.
"Do you have to put polish on after every time you go out?" Charles asked Trooper Thomas O'Mara, before asking Trooper James Fisher about his knee-high ceremonial boots, saying: "How do you walk in those?"
"It's very difficult, sir," replied the soldier.