GeckoLib

GeckoLib is a library mod created by Gecko. An animation library for Minecraft mods that enables complex entity animations created in Blockbench. As a library mod it concentrates on shared code that other mods build on, and it has been downloaded around 180 million times, which puts it among the options players reach for in this space.

GeckoLib is built for players installing mods that rely on it for advanced animations. GeckoLib is an animation engine many creature and machine mods use for smooth, keyframed models, so you usually install it because another mod requires it. It is a deep, systems-heavy mod, and getting the most from it means investing real time in its mechanics.

The mod is available for Forge, Fabric, NeoForge, and Quilt, so it fits a range of modpacks and launchers. It currently supports Minecraft 1.20.1, 1.20.4, and 1.21.1, spanning the 1.20.1 to 1.21.1 releases. It runs on both the client and the server, so on a multiplayer server every player who connects needs it installed alongside the server itself.

Because it ships for both Fabric and Forge-family loaders, it slots into most popular modpacks regardless of which side they are built on.

As a library mod, GeckoLib adds no content of its own and has a negligible footprint, it exists to support the mods that depend on it.

If you are comparing options, a similar mod worth a look is (no direct alternative, it is a dependency library). They overlap in purpose, so most players pick whichever suits their loader and pack.