Welcome to Deathly Creatures! This is a Side Project I'm working on since I feel like not a lot of Mobs mods add a true challenge to early/mid game whilst keeping simplicity AND adding new things. It's always too much in one direction. The project is actively being made, so let's get into what's already in the mod:
The Contact Creeper is a hostile mob that spawns at night in the overworld or in caves(like most mobs in this mod.) Instead of charging up like a normal Creeper, it will only explode when it comes in within a block's reach of you, but there will be no wind up. It instantly explodes. It's detection range is shorter than a normal Creeper's, but it also moves faster to compensate. It drops gunpowder, TNT, and redstone dust.
The Tower Marrow is a new monster that spawns rarely. When it first sees an attack target, 4 Spider Skull mobs will dig out of the ground and attack the target, slowly dying off after. It will jump up and slam itself into the ground, creating a damaging shockwave. Killing this abomination enough times can reward you with some of the bones tangled in it's body, to craft a powerful AOE weapon or a special kind of shovel.
Rarely, in the overworld, you will find an Infectyceps fungus. When a Zombie from the overworld touches it, it will convert into an Infectyceps Zombie and some of it's flest will fall right off it's body. It will infect other Zombies with the fungus, turning them into a way more powerful version of itself that poisons the target and moves faster. These creatures will NOT despawn, making it important to deal with them quickly if you find one. On death, they explode into a cloud of poison, effecting anything around it's area of demise. 
The Stone Golem is a stone force to be reckoned with, dealing high melee damage and knocking you away from it. It has a chance to stun you when it attacks, rendering you unable to move. It seems hard to deal with, but there is counterplay. If you can put it in water, or splash a water bucket over it's head, it will take damage, lose it's ability to stun you, move slower, and take slow damage over time.
This little guy is an outlier, he is the Emberling. They spawn naturally in the Nether extremely uncommonly, and catching one is very difficult. If you get too close, he will erupt into a flash of dangerous flames. Coat a glass bottle in magma cream to get a Heat Resistant bottle, and lure the skittish creature in before scooping him into the bottle. With the new Blazing Bottle, you can enhance Fire Resistance potions in a brewing stand to create an incredibly long lasting fire resistance brew, or you can make a new item that puts a custom enchantment on your weapons. Incineration Aspect is incompatible with Fire Aspect and serves as an alternative, dealing AOE damage that burns the main target and the surroundings with a high damage strike.
This mod requires GeckoLib to run, and it will recieve updates in the future.