Emperor

An Emperor is a male monarch that is the masculine equivalent of an Empress.

Friendship is Magic
Long before it became known as Equestria, the land of the Ponies was ruled over by the Sheep Emperor Grogar, also known as the Father of Monsters for unleashing such creatures upon the inhabitants. His rule was eventually brought to an end by Gusty the Great, who stole his Bewitching Bell and with it a large portion of his powers. The city of Trot was ruled over by Emperor Incitatus, though the city was later invaded by  and her Changeling army. In an alternate reality, a version of Prince Bluebood proclaimed himself Emperor of the moon after being banished there.

Sombra is known to have declared himself, or else been declared by others, an Emperor on at least two separate occasions. The first was during the Siege of the Crystal Empire, after he unleashed his fellow Umbrum and briefly set his sights on conquering Griffonstone and Yakyakistan. However, after realizing that he was in control of his own destiny, he abandoned his villainous pursuit of conquest along with all royal titles. Later, after being brought forth by Grogar, an evil Sombra was proclaimed emperor as he conquered Canterlot; however, he was subsequently defeated by the Mane Six.

"Kings" appear to be the equivalent of emperors, though in some cases they are implied to be inferior.

Friendship is Magic

 * Emperor Incitatus (Trot)
 * (Equestria-formerly)
 * Sombra (Crystal Empire/Equestria-formerly)
 * Prince Blueblood (Deviations) (Moon (Deviations))