From the Super Mario Wiki
The Mushroom Kings are the monarchs of the various lands of the Mushroom World. They are first found in Super Mario Bros. 3. There are seven Mushroom Kings in total, and each was turned into a different creature by Bowser's children. The Kings do not have official names, although The Adventures of Super Mario Bros. 3 gave some of them names, and The Super Mario Bros. Super Show! even introduced several Mushroom King-like characters and their territories. Like Princess Toadstool, they have Toad helpers.
 Super Mario Bros. 3
Not long ago, the evil Koopalings invaded the seven kingdoms of the Mushroom World. Each Koopaling came to the castle of the land and knocked out the Toad, then stomped the Mushroom King to steal his Magic Wand and transformed him into a creature. The evil Koopaling then ruled over the world in the king's place, the king being in his vunerable state, with Boom Boom and the Koopa Troop all over the kingdoms, until Mario and Luigi, sent by the princess of the Mushroom Kingdom came to help them, first starting with Grass Land.
To save a Mushroom King, the Mario Bros. have to go to his castle, where the Toad is pleading the heroes to save the world and return the Mushroom King back to the way he was, with the Magic Wand the Koopaling stole from him. After that, Mario and Luigi must defeat the Koopaling, destroy the airship and return the Mushroom King back into his human shape. As a Mushroom King, he thanks the Mario Bros. for saving him, he gives a letter from Princess Toadstool to Mario and receives a special item. Mario and Luigi may continue their way to the next kingdom of Dark Land after saving the Pipe Maze King. If the player manages to restore the Kings to their human forms while wearing a special suit, then they get a different answer for each different suit. With the Tanooki Suit, the King says "Thank you kind racoon! Please tell me your name?" With the Hammer suit he asks if they could lend him their clothes. With the Frog Suit, he asks if they want to be restored to their original form, thinking that they had been transformed into actual frogs.
In the GBA remake, Super Mario Advance 4, if the player defeated Bowser before saving the kings, upon selecting the castle, the player is treated to a cutscene where it shows the Koopaling of the respective world invading the castle, snatching the Magic Scepter, using it to transform the king, and then escaping seconds before Mario arrives. In addition, because Princess Toadstool would have been saved beforehand, the King will not give the player a letter from her.
Each Mushroom Kings had been transformed into strange creatures by the Koopalings:
 The Adventures of Super Mario Bros. 3
The Adventures of Super Mario Bros. 3 created personalities for several of the Mushroom Kings. Many, if not all, of them also looked different.
The following screenshots are from the Super Mario All-Stars version of Super Mario Bros. 3.
 Mushroom Kings
 Transformed Mushroom Kings