Send massage.
Grogar was very much real. Every villain (sans Cozy) has heard tales of him. Then we see his actual bell, and it has the same yellow-and-black electrical aura that Discord imitated. Then the trio finds a book that explains how it works. And then obviously Discord gets hit with it, and says his magic’s trapped ‘inside Grogar’s bell’.
So yeah, actual historical figure. IS he still around? Dunno.
Also, keep in mind, this is Equestria. Every legend turns out to be true.