Your buddy is YOU. It's a unstable form of life. Installing this mod will enable that energy to materialize and spawn when you log into a world.
as stated above : "depending on what item is used on your buddy, he will change to something different." Meaning the unstable life form of your Buddy can be altered depending on what item you use on it. Much like Eevee, the Pokemon.
After doing some decent research, we found that the pet buddy is a part of the God Entity. (no, it is not Notch, Notch was our very first creator and guards our lands). The God Entity possess the genetic composition of all entities in minecraft. Creepers, ghasts, pigs, villagers, ... .
The shrines we discovered contain essence of the God Entity, and can be transfered into small statues that we found in chests close to the first Shrine. When used, these statues concentrate their energy into a small form of life. The statue only containes enough energy to produce a small entity not bigger then a few inches, yet loyal to him who called him to life. His dna and energy are very unstable and he can therefor be changed body by the slightest touch of an item that may contain dna of any entity in minecraft !
Maybe due to him, we might ever be able to find the God Entity, or even recreate him, allowing us to discover the history of evolution in minecraft