DECI-lib is not just a library mod, it allows you as a datapack creator to make entities entirely from scratch using an intuitive .json structure. This framework allows you to select from many deeply parameterizable and configurable behaviors, with flagship behaviors such as ambush, burrow, and flee, as options you can deeply customize. You can rely Azure/Geckolib to render the mobs, or you can rely on the mod's category based rendering, based on parameters such as bruiser/ambusher/skirmisher and quadruped/biped. All mobs are registered when the game loads, and their spawn rates can be configured as well so that they are instantly able to be spawned in the world.
If you need assets to go along with the mobs, you can view the asset library I released a few days ago in this forum.
DECI-lib will be releasing TODAY!