Grogar being a dark magical ram being that was cast into the shadow realm thousand of years ago, going with the idea that Grogar ate the Hito-Hito (Human) Devil Fruit and thus capable of using Chopper’s Rumble Ball to access his stronger ram abilities.
Thought of the idea for Chopper to face an opponent that has his original skill set and thus must “evolve” to the newer forms he has currently uses post-two-year skip.
Grogar as a remnant of an opposing group against Canterlot, plans of using both the magical power of Equestia and the physical abilities granted to him from the Devil Fruit.
Grogar using Chopper’s own techniques has left the reindeer to resolve into evolving his forms against someone who is using his old skill set. Chopper hopes that the Elements of Harmony could neutralize Grogar’s magical powers so he could face him in a physical rematch.
Here are two of his forms Climb Point and Ram Point (counter to Chopper’s Jump and Horn Point)