So, I've been thinking. With entity plugins working now, I think it's time to bring up the question Is it possible to import custom mobs, how and what can I do to execute this? If not, how come? I've always wanted to ask this, and I finally decided to ask the pmmp forums I have no experience in CAD, or coding in PHP but it never hurts to ask.
You can register and create a entity, and then use texture packs. (That's what I think you're trying to ask)
You can make 'custom mobs' by spawning a Human entity (or one that extends it) and changing the geometry into a model that you want, and you can use a skin to have the desired colours. We sadly cannot change the model of other entities than humans. (Yet)