Actually, the TOS states that mods/plugins(not referring to bukkit)/whatever-you-want-to-call-them are intellectual property of the creators, only the code for Minecraft belongs to mojang.
TOS
So, Mo' Creatures belongs to Dr.Zhark, however with a polite request to him I'm sure he will allow it, I asked him back around 1.5 if it would be alright if I port it to work with modloadermp and he said yes.
Can you please site where mojang says minecraft is opensource?