This is "A Boy holding a Grey Horse" by Abraham van Calraet. Painted in the Dutch tradition, we see a young stable boy holding onto the reins of a majestic grey horse. The horse dominates the composition, its forequarters standing out brightly against the dark hillside while its rump is contrasted against the luminous sky.
The dramatic light in which the scene is presented creates a sense of grandeur and emphasizes the size of the animal. Notice how the horse's presence turns the heads of two passers-by in the background - perhaps they're the owners, or simply admiring the beauty of this magnificent creature.
Interestingly, Abraham van Calraet wasn't the only artist to paint versions of this scene. There are several other versions, including one in the Wallace Collection, and another that's thought to be by Abraham Pietersz. van Calraet himself. Some art historians even believe a version by Aelbert Cuyp, a famous Dutch painter from the same period, may have been faked to increase its value.
The painting is made with oil on wood, typical of many 17th-century works of art from this region. Unfortunately, we don't know the exact date it was painted, but it's likely to be somewhere between 1670 and 1722.
A Boy holding a Grey Horse hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.