Quote from Writr
There is already a program (I believe it is Techne) that allows you to make very good Minecraft models, for both items and mobs.
As for implementing them into Minecraft itself, that is a bit more complicated and involves coding. I believe there was a very basic program that existed for a while, but it did not allow you to have unique behavior; any mobs you created would have the same basic behaviors of the current Minecraft mobs.
Personally, as much as people want this sort of thing, I feel that it would ruin Minecraft. We don't need Minecraft to become Spore.
What is a spore and why would it ruin minecraft?