Tyrant

A mini-boss that spawns in the Nether. I imagine they spawn in an arena room in one of the structures in the nether like a bastion or fortress. They are large (4 or five blocks tall) mobs. It would be a dark red humanoid with large goat horns and scars all over its body. Its face is covered by a metal helmet with only its glowing yellow eyes visible. It has sparse armor like a shoulder pad and iron boots. It wields a large netherite sword and a stone shield that can't be disabled with an axe. He also has a skeletal tail that uses to swipe at any enemies behind him. 

The way this fight works is like a duel. If you use too many projectiles he will start to block. Continue this and he will summon two blazes so he can go in on you. The fashion to fight the tyrant is with a rhythm: approach him to bait a sword swing, get him to miss, and get a hit in as he recovers for he has relatively slow attack animations. He has a few attacks: a swing that you can jump over, a downward slash that should be strafed, a forward stab with lots of reach, a tail swing to get enemies off its back, an can summon blazes. Maybe it can have some projectile like a fireball or ground shockwave. 

On death it drops lots of experience, and netherite equipment that all have either binding or vanishing on them with low durability. Maybe it can drop some new, exclusive item, although I have no idea as to what. 

I imagine this being a fast paced fight with an A.I. that doesn't just chase the player.