I just finished experimenting with this. Here's my repository if you want to have a look: https://github.com/TheMikeste1/Wabbits
You'll need to register an EntityType. Please note that the way I'm doing this is a little strangely because of the way I set up my spawn eggs. There's a lot of code you don't need in my repository. Once you've registered an EntityType, you'll need a probably need a custom entity class. For rendering, you'll need a renderer, renderer factory, and a model (unless you want to use one of the existing models. Once you've made those, you can register your renderer like so (note that this is client side only!):
RenderingRegistry.registerEntityRenderingHandler(WabbitEntity.class, RenderWabbitFactory.INSTANCE);