This should be as simple a matter as the standard player renderer layers. For Mystical World, the renderer layer should still be implemented in Roots. This may involve switching to a "base" render layer and then subclassing it based on the type of entity.

As it requires a model that's slightly larger, either there can be the bulk creation of the render layers at post-init (or wherever) with reflection used to recreate models using a slightly large scale (if the model supports it), or, the option I'm currently preferring (as there's no way to really identify other "familiars" if they don't implement EntityTameable) of hard-coding.

The latter will have implications on cross-mod compatibility.

• Horses
  • EntityDonkey
  • EntityHorse
  • EntityLlama
  • EntityMule
  • EntitySkeletonHorse
  • EntityZombieHorse
• Tameables
  • EntityOcelot
  • EntityParrot
  • EntityWolf
• Mystical World
  • EntityBeetle
  • EntityFox
  • EntityLavaCat

I've decided not to do this and instead just use the particle effect. Messing with the renderers is really too painful.