Ba's Cave Enemies currently adds 4 mushrooms and 4 mobs to caves. The mobs are rather rare / situational to come across and don't break the vanilla immersion in my opinion. This mod will be updated with more mobs when I feel like it. Also, if you find anything unbalanced feel free to leave a comment.

This mod requires GeckoLib.

The mobs as of now are:

Troglodyte

Small creatures that inhabit lush caves, traversing them in small groups or alone. They lay eggs after killing zombies. They will also attack you once they deem you weak enough. Under the same conditions, they may also kill Feruminops.

Feruminops

Small trilobites that inhabit lush caves—not defenseless.

Cave Shark

Yet another reason to avoid underwater caves. But if you do venture in, remember: not every blue light comes from a glow squid. Might be worth hunting for their teeth, which can be used to brew potions of buoyancy.

Snooper

A sneaky little creature that ambushes its foes. It specializes in killing Endermen but also targets Illagers and players for some reason. Active ears will help against them.

Secret Troglodyte

After killing a Feruminops, Troglodytes lay special dark eggs, from which these player-friendly Troglodytes hatch.