Although the identity of the sitter is unknown his elegant dress and bearing suggest that he was an individual of wealth and distinction. The inclusion of a hunting dog was quite common in portraits of aristocrats, and the gold chains are a usual sign of honor. The suggestion of a military identification is enhanced by the gesture of his hand fisted at his waist, and the standing, three-quarter-length pose was generally used by Mor in his paintings of aristocrats as opposed to the more informal poses he used in his likenesses of middle-class subjects.
The most likely precedent for this painting is the Portrait of Charles V (Prado, Madrid), done in 1532 or 1533 by the Venetian artist Titian. From Titian, Mor adapted the compositional arrangement for his depiction of a standing man with a dog. Similarly, the way that light is employed, brilliantly illuminating selective portions of the figure while arbitrarily obscuring other parts in dark shadow, is thoroughly Titianesque. Mor's style also reveals his training in his native Flanders, in his close attention to detail and delight in depicting textures.
The deft handling of paint and the astute psychological presentation clearly demonstrate why Mor was such a sought-after portraitist during the sixteenth century, anticipating the achievements of the great portraitist of the aristocracy in the following century, Anthony van Dyck.
More information on this painting can be found in the Gallery publication Early Netherlandish Painting, which is available as a free PDF https://www.nga.gov/content/dam/ngaweb/research/publications/pdfs/early-netherlandish-painting.pdf