Donkey Kong - Donkey Kong was Mario's first Boss, in the game, he captured Pauline and took her up to the top of a construction site where Mario would then have to go up and save her. Donkey Kong would start to throw multiple barrels at Mario whom would have to jump over them to get past. Once Mario would hit a lever, Donkey Kong would drop and get hurt where you would then have to repeat.
Donkey Kong Jr.
Mario - This is the first and only game where Mario is the boss. In this game, Mario captured Donkey Kong, and put him in a cage. Now, Donkey Kong Jr. would have to go through multiple levels trying to save his dad. Mario would send out animals to stop DK Jr. Once you saved Donkey Kong, he would kick Mario out of the way.
Donkey Kong 3
Donkey Kong - Donkey Kong appears once again as a boss that, instead of Mario, Stanley the Bugman must defeat. In the game, Donkey Kong has gotten into Stanley's Bug House and is letting all the wasps out, now Stanley must not only defeat Donkey Kong, but also kill all the bugs with his Bugspray.
Super Mario Bros.
Bowser - At the end of each world, a fake bowser would be waiting for you, but then at the end of the game, the real Bowser would be there. In the game, Bowser kidnapped Peach, and turned all the Toads into stone while hoping to become king of the Mushroom Kingdom. He sent out a huge troop to defeat Mario and Luigi, and also put disguses on some of them to look like Bowser. At the end, Mario would face the real one, whom would jump, blow fire, and even throw hammers all at the same time. Once you got past him, you'd hit the switch to drop the bridge making him go into a pit of fire.
Super Mario Bros. 2
Birdo - Birdo appears multiple times in the game as a minor boss that will shoot eggs at you. Mario, Luigi, Peach, or Toad will then have to jump onto the egg, pick it up, and throw it back at her. Like all the other bosses, she is a minion to Wart, the evil frog like creature.
Mouser - Mouser is a minion to Wart whom tries to kill the Mario Bros. by throwing bombs at them.
Tryclyde - Triclyde is a three headed snake whom, like every one else is a minion to Wart.
Boom Boom - Boom Boom appears once in every world other than the last, where he appears a few times. He is a minion to Bowser, and a miniboss usually in the middle of each world. Each time you fight him, he will become much better, such as the ability to grow wings and fly.