King

A King is a male monarch that is the masculine equivalent of a Queen. In some circumstances, a king is the father of a Prince or Princess. Equivalent and similar titles for King depending on the culture include Masters and Emperors.

My Little Pony
Prince Rudolph was the son of the king of his land. King Kobb ruled over the Cavern Shore.

Tales
The Isle of Pony was a kingdom located somewhere far from. It was ruled by a king and Queen respectively. Rosey, friend of turned out to be their long lost daughter and joined their royal family.

Friendship is Magic
The kingdom of Griffonstone was established by King Grover, who used the magical Crown of Grover to achieve his rule and instilled pride into the Griffons with the Idol of Boreas. The line of Griffon Monarchy continued through fourteen kings, ending with King Guto. During Guto's reign, the idol was stolen by an Arimaspi and lost in the Abyssmal Abyss. The Crown of Grover later ended in the possession of Princess Celestia through unknown means. The Storm King also established himself as ruler of a large domain, though he was interested more in acquiring power than in actually extending the borders of his realm.

Celestia's cousin King Leo also came to serve as monarch of Aquastria.

"Emperors" appear to be the equivalent of kings, or in some cases superior.

IDW
One of the oldest known kings was King Anubis of the (presumed) Anugyptian kingdom, a tyrant ruler who reigned over both his own Jackal people and the enslaved Pony population of the area. Just prior to the Founding of Equestria, the tribe was ruled over by King Bullion, while the  city of Timbucktu was ruled over by King Orion before it fell to. The Infernal Lands were ruled by the Centaur King Vorak at about this time, while the rule of the Crystal Empire was briefly usurped by the Umbrum King Sombra. By contrast, an alternate universe version of Equestria was justly governed by Sombra's benevolent Unicorn counterpart, until he drained the corruption from the Princesses Celestia and Luna of his universe to save his reality and that of Princess Celestia.

The Deer King Aspen rules over the Kingdom of Thicket. In The Mysterious South, the kingdom of Abyssinia was ruled by a ruling family of both king and queen, both of whom seemed equal, though the queen's more assertive, brave and intelligent personality may have made her the "dominant" monarch.

My Little Pony

 * King Charlatan (Penguins)
 * King of Magic world (Magic world)
 * King Kobb (Cavern Shore)

Tales

 * Squire's father (Squire's kingdom)
 * King of Isle of Pony (Isle of Pony)

Friendship is Magic

 * King Anubis (Anugypt)
 * King Bullion (Unicornia)
 * King Vorak (Infernal Lands)
 * (Crystal Empire)
 * Mirror-Sombra ($𝜇$ (Mirror Universe))
 * King Orion (Timbucktu)
 * King Grover (Griffonstone
 * King Guto (Griffonstone)
 * King Longhorn (Cattle Rustlers)
 * King Aspen (Kingdom of Thicket)
 * Storm King (Storm King's Realm)
 * Abyssinian King (Abyssinia)