For years I have struggled with different Animation APIs. AnimationsAPI by thehippomaster was a huge step but just isn't enoguh (shoutouts to him, he got me to this). With this mod you can create difficult animations in about 20 Minutes time.
What's so different about this mod compared to other is that you have the possibility - you are not forced to - to make your mobs and animations swappable with resource packs. Not only textures, the whole mob can be replaced (bounding boxes will currently not be replaced, that's WIP). That allows your users to create their own experience. You can find the mod on the forum and read more about it on the wiki.
Requirement: The newest Forge installation for the Minecraft version you play at