The Lady Knight in Child Of The Storm, a human woman (albeit immortal) who hasn't appeared on screen, but has cast a shadow of proceedings. She taught most of the major badasses in history how to fight. Cuchulainn? Taught him. The Argonauts? Taught most of them. Real LifeBadass William Marshal, the greatest knight? Taught him too. She even took Sif and Fandral under her wing and turned them into the Goddess of War and the finest swordsman in the Nine Realms with only Sif for a peer and one of the deadliest men in the entire universe. She was also the best of Arthur's court. Yes, even better than Lancelot. No one really knows anything about her, beyond the fact that she's epically badass, trapped on The Slow Path and probably something of a Hurting Hero by now - though she hasn't been seen since the 18th century.
And more prominently, the Winter Soldier, who, by his reputation, is basically invincible and pretty much the entire cast, both good and evil, is either starkly terrified of him, or, at the very least, regard him with a kind of wary respect. And, pretty much every time he makes an appearance, he very much lives up to his rep.
Mr. Black in Make a Wish. "Whoever caught the rabbit first would get the title of the best and be given the job of tracking down all the death munchers. Umbitch took one look at the forest and concluded that rabbits don't exist. The Aurors went in with fifty men and came out with ten . . . having fought a great battle with the rabbit and barely escaped with their lives. And Mr. Black . . . he didn't even bother to show up . . . three days later, the rabbit died . . . of 'natural causes.'"
Don't you mean that the rabbit died . . . in an 'accident'"
"Yeah! Twilight Sparkle!" The gryphon went on, oblivious to Twilight's nervousness and Pinkie's Pinkieness. "She's an alicorn sorceress from Manehattan. Purple, kinda like you." The large gryphon leaned her head down and lowered her voice, conspiratorially. "Rumor going around the aeries is that she's actually another sister of Celestia and Luna and the Manehattan story is just a cover. You really didn't know about this?"