I have a lot more experience regarding this mod in 1.12 (due to using/having used it in packs in this version), but I did try again its 1.20 iteration, screenshots from there

What is it ?

This is a mod focused on adding tons of variants to existing monsters in a similar way to the 1.10 update adding the strays and the husks as undead variants.

What does it offer ?

When spawning, vanilla mobs now have to chance to get replaced by their default special mobs counterpart (which are called the "vanilla replacement" mobs -> for example a minecraft:zombie spawning has a high chance to get replaced by a "specialmobs:specialzombie ; this feature can be disabled if you have mods in your pack which only affect specifically the vanilla mobs), or less often replaced by special variants (depending on the biome, dimension, base rarity and possibly other factors which can be controlled; for example, in plains a zombie spawning has a high chance of becoming a specialmobs:specialzombie but can instead spawn as a specialmobs:zombiebrute for example).
The in-game config is very clear and these informations have been obtained there.
It is by tweaking the base chances and exceptions that you can make stuff such as variants which appear more often (or even always) than the vanilla replacement (such as plague zombies appearing more often than the vanilla replacement for zombies when appearing in swamps, but making plague zombies have a base chance of 0 to make it so they end up being swamp-specific).

Here's a list of the affected mobs (with examples of the most interesting variants; though there's a lot more which I won't talk about) :

When several vanilla mobs are set within a common category it means they share many variant concepts

Creepers :

-The dark creeper which destroys perfectly well sources of light while giving you blindness
-The Scope creeper, looks hilarious; it's a post 1.12 mob which has spyglasses on his eyes, he can detect you from much further

Zombies , Drowneds & Zombified Piglins :

-for zombies there's the mad scientist (only available in recent versions), which poison you in melee
-for drowneds there's the exclusive abyssal (which is more resilient and give you a weight effect to make you sink faster and tropical (which throws poisonous tridents at you) variants
-for zombified piglins the most special is the Vampire Piglin, which is really resilient and looks closer to a regular piglin than an undead

Skeletons & Wither Skeletons :

-There's the ninja, which hides by turning into blocks and tries to backstab you, exists for both
-There's the pirate (which is new too), he can appear at sea (can be changed) and can use boat+shoots spectral arrows

Slimes & Magma Cubes :

-The Blackberry and Volatile , are acting like creepers
-The newer Potion Slime , can give you a potion effect on melee depending on the potion color it has

Spiders & Cave Spiders :

-The Witch Spider can give you negative potion effects on melee, while giving itself positive ones for every hit she gives (behaves differently in 1.12)
-The Water Spider can walk on water (appears on the sea and rivers by default too (it's a new mob)
-The Flying Spider can jump really far to reach you quickly

Silverfish :

-There's the Fishing silverfish , acting like some other fishing type mob variants and having the ability to reel you in
-There's the "pufferfish" (new), which can poison you, looks like a pufferfish and can swim too
-There's the Albino silverfish which can shoot you with icy projectiles

Endermen :

-There's the Mirage Enderman which makes you see double as it appears to clone itself
-There's the Thief Enderman which can teleport you when hitting you
-There's the Runic Enderman which can shoot a laser of its mouth and make you levitate on hit (is new)

Witches :

I will go in details about all of their variants since I find them to be the most detailled and interesting (plus there is less variants for them than for most other groups)
-Witch of Domination throws homemade levitation potions, and can use some telekinetic powers to move you around; she has a pretty stylish costume and a nice hat
-Ice Witch can turn near water into ice, allowing her to walk on it, she is also pretty resilient; she wears a blue coat (is new)
-Witch of Shadows throws potions of blindness/wither; she looks ominous- Lichthrows instant damage potions (exclusively) and sometimes throws skeletons at you too; she looks like a zombified witch- Witch of the Wildsthrows Poisonous potions exclusively, can summon little spiders and can summon a larger one to ride too- Wind Witchcan teleport like endermen, which makes her probably the hardest mob to fight### Blazes:
-There's the Joltwhich will teleport every time you hit it, summoning lightning in its place-The Armored blaze(new) has an armor, which needs to be destroyed before hitting the entity hiding behind### Ghasts:
-the Queen and King ghastsare more powerfull versions of ghasts, one summoning little ghasts while the other has more firepower-the Slab ghast(new) is much faster and aggressive than his friends-the Unholy ghastis really resilient, stalks you but attacks in melee if you get too closeMost of the mobs's models were unchanged back in 1.12 (only having texture tweaks or recolors), but the mod has since updated , adding new mob variants and readapting older ones, even changing the models of some creatures, the most notable being the witches|| in terms of aesthetic improvements.
Other tweaks were made to general undeads, which can now appear naturally wielding bows (or sometimes skeletons will appear holding and using swords), and can appear with and use shields (which will negate all projectiles before them and some melee frontal hits).

Main issue with this mod ? The performances .
As it turns out, the way this mods checks for mobs to replace by other mobs seems to make the game ever so slightly laggier every time a new concerned creature appears. it's actually because certain mobs's AI causes this, it doesn't seem to apply to all mob replacement
it's small, but noticeable in larger packs.

In conclusion

I like this mod, it does its job at adding vanilla-like mob variant similar to the husk and stray in looks and behavior.

Quite configurable as you can decide exactly in which biomes/biome types the mob replacement occurs.

Unfortunately, its impact on the performances (especially in 1.12) land this mod in a similar spot to Quality Tools in the sense that it cannot be used in serious larger packs .
Maybe the performances issues will get fixed somehow, or are already fixed in the very recent 1.20.1 update and I just haven't played enough to notice.
take it with a grain of salt as I remind you I have mostly played through the 1.12 version