There is a mod that allows you to do that, it just requires a bit of work.. it isn't like the Tails mod or More Player Models. It allows you to implement 3D models, such as 3D tails and ears, like Aphmau does it (Aphmau uses a custom mod made by her crew, but this is the same idea), onto your Minecraft Character.
Mods: BlockBuster & Metamorph
It takes a bit of work but it looks good. You need Blender and some other things.
Link to a Youtube Video on how to do this:
iiRaeRae, The problem is that they still don't have animations. It isn't that we haven't figured out how to use BlockBuster and MetaMorph to make the object models, it's that we can't figure out how to make them animated in MineCraft.
I know this topic is very old, but I want to say that I am also in the works of 3D ears and tails mod. I'll also make sure it'll be compatible with another mod, the More Player Models mod. So, if the model is tall/short, the ears will stick to the body. The ears and tail might not be animated, though, but I'll try my best.
Also, reminder, mods take a long time to make, so please do not message me, telling me to hurry up, threaten me or anything regarding the mod. I will post updates on how it's going so you all know.