It was Mary of Austria, Queen of Hungary who arranged for Titian to carry out this work and it is believed that Charles V was fond of Titian's artistic style and also the way in which he would depict his subjects in a flattering finished painting. The Emperor had confidence that Titian would produce something which made him look a strong and successful ruler.
The two built a surprisingly close friendship when considering Titian's relatively lowly background. The artist was treated very well by the Emperor's staff, under his instruction, whenever he would visit to produce a new portrait. Whilst not being of high stock, Titian was clealy an extraordinary artist who also had a charming, humorous personality which helped him to build significant relationships throughout his career.
The artist constructs this image of a strong ruler with several subtle touches in the painting, namely the way in which the horse is strong, but comfortably in control by this experienced horseman. His armour is also resplendent, and remains on display at the Royal Armoury in Madrid, whilst the original painting can be found at the Museo del Prado.
Anthony van Dyck later produced a Portrait of Charles V on Horseback in 1620 which offered an intriguing alternative style to the same topic. Van Dyck was more aggressive with the lighting, leaving a more dramatic scene. There is again a close bond between man and horse forging a close partnership that you imagine would have been build up over years of battle.
Equestrian portraits were frequent during the Renaissance and Baroque art periods. Some other fine exponents of this very specific genre included Diego Velazquez with the Count Duke of Olivares, Elisabeth of France and Philip IV.
The most famous horse depiction of all time is most probably Whistlejacket by George Stubbs, a British artist who went to extraordinary lengths in order to ensure the accuracy of his horse portraits. Other points of interest include The Four Horsemen of the Apocalypse by Albrecht Durer, An Arab Horseman at the Gallop by Eugene Delacroix and Etude of Horse by Peter Paul Rubens.