Brainstorm and Jack themselves have evil counterparts in the form of Thunderstorm and Shadow, who are one of the series' most competent villains.
Dungeon Keeper Ami has Mercury and Mukrezar - both are physically weak, but possess devastating magical skill, both conquered the Avatar islands, both are resourceful and cunning and use that to win rather than brute strength.
Fallout: Equestria has Lilpip and Red Eye. Both are Stable Dwellers that wish to make Equestria like, or better than what it was before the Great War. They also both represent one of the Elements of Harmony when corrupted.
Red Eye, who gave up every comfort he had to bring some sense of order to the Wasteland, represents corrupted generosity.
Lilpip, who's overwhelming concern for others often leads to self-destructive and irresponsible decisions, represents corrupted kindness.
Lilpip herself lampshades this as she realizes that, though she despises Red Eye's system being built on slavery, it's the first real working infrastructure Equestria has had in hundreds of years. Red Eye, for his part, feels enough mutuality with Lilpip that he offers to abdicate his position to her. It doesn't help that Lilpip is your typical Fallout protagonist (albeit one with exceptionally good karma) and Red Eye is very much an anti-villain compared to other antagonists in the story.
Clash of the Elements: In part 2's finale, Chaos Alex is this to Alex Whiter. Though this is actually Inverted from Chaos Alex's point of view, for he thinks the real Alex is the evil half. Also, the Sin Samurais to the Genesis Samurais.
Eridan ends up becoming this to Nepeta in Herding Cats. Where Nepeta ships her fellow trolls in redrom, Eridan does so in blackrom.
Racer And The Geek does an interesting take on this trope. Service in the Royal Guard or the Equestrian Royal Army, is considered highly honorable by the populace. However, those who do similar work in a private outfit are considered the lowest of the low. Regardless of the actual character of those involved, their actions and motives will be interpreted very differently depending on if they are a trooper or a contractor.
Thanks Kyubey had Juubey as Kyubey's evil counterpart, by having Kyubey have actual emotions and being more forthcoming with information about Puella Magi, and having Juubey act like...well, the canon Kyubey. Juubey got better.
The Zarner siblings, Takko, Jakko, and Zot started out as evil counterparts of Yakko, Wakko, and Dot. They first appeared in a parody of The Powerpuff Girls, where they were created by Pinky and the Brain as part of a plot to Take Over the World. This was their only Season 1 appearance. Even though the Zarners started out this way, they've become less evil starting with their second appearance. They even made it into the theme song! (In Season 1, the 4th section started with "Meet ZakKo, Pakko, Makko, Jot, the Yarners, and Coach Nurse." From Season 2 onward, it went "Meet the Warner cousins, Yarner Twins, the Zarners, and Coach Nurse.") By Season 3, the Zarners have become good friends of the Warners.