A remake of an old favorite, Fuzs has updated the Mutant Beasts mod for modern versions of Minecraft for Forge and Fabric.
This mod adds in several boss versions of vanilla mobs that have a small chance to spawn instead of their standard counterparts. These boss mobs are surprisingly well made and when killed they drop powerful albeit gimmicky weapons. This mod also has a good config allowing you to change the spawn rate of each mob.
In my personal packs I like to turn down most of the mutants' odds of spawning in order to make each encounter feel more special and so I'm less likely to get bodied in the early game.
There's no way "Grassington" is a real place right? Right?!