Intimidating/Terrifying Antagonists?

I need help with making an intimidating antagonist for a Fallout: Equestria fic I am writing. In terms of looks, this guy is supposed to be massive. I’m still working out demeanor and all that stuff, but I’d like some input on how to make an antagonist that is terrifying.

Hm, so are you like, going for an intelligent Super Mutant Behemoth pony?
 
I ain’t too experienced writing good antagonists, but as long as his motives are sinister and reveal themselves little by little along the way, that’s a start. How one affects the environment can be a good way to flesh out an antagonist’s terror. You could also give them a creepy tic or obsession with something material, like dolls or what not.

@Sono  
Not really a Super Mutant Pony, just a massive earth pony or something.
 
http://derpibooru.org/81905?scope=scpe15901fdaa74405a81b68ba697373c5ad00f18417
 
Kinda like this dude.
 
(Curse my lack of link and/or image embed knowledge.)

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@Background Pony  
No.