With so many quest and bosses to fight, it is not that surprising that we would sometimes miss out on a few mini-bosses here and there. If you plan to go back in and hunt them down, here are some of the minibosses that you might have missed.
One of the minibosses that you might have missed is Vulthuryroi. The dragon can be found in Blackreach. You will need to summon it using the Unrelenting Force shout that the lamp about you. You can choose not to fight with him after summoning him, he will kill everything in Blackreach for you.
Those looking for a challenge might want to seek out the Ebony Warrior. It is by far the most secretive minibosses. The Ebony Warrior will seek out the Dragonborn when he hits level 80 and then the players will have to complete the quest to seek him out. Not only is he hard to find but he is also hard to defeat as well. Many players said that he is easily one of the hardest minibosses to fight in the game. The bright side is that you will get to loot his powerful armour and weapons once it is over.