Grumlok and Gazbag
They had a model which was included in the special edition set, but their canonicity is dubious. They most likely existed in Fantasy but with a different storyline.
Grumlok is an Orc, leader of the Bloody Sun Boyz tribe. He was more cunning and brutal (or brutal and cunning?) than your average Boss, but still struggled to form a WAAAGH! due to his single-minded determination to wipe out a foe one at a time rather than focusing on a bigger picture (or even seeing it). Unusually for an Orc, Grumlok actually had a "friend"; the Goblin Shaman Gazbag, rather than being some weedy annoying git he pushed around to get shit done like most Goblins are to an Orc, was his constant companion who rode his shoulders or back. For years the Bloody Suns and their twin leaders held the center of the Badlands, putting down all challenges but being unable to inspire loyalty and take command of their rival tribes.
Meanwhile in Naggaroth, Malekith had begun a far-reaching plan to use the forces of Chaos to wipe out the Empire and the greenskins to wipe out the Dwarfs so he could destroy the High Elves and take Ulthuan. He knew he needed greenskin pawns, so he sent his minions out to identify and kidnap the supreme greenskin. They returned with Grumlok and Gazbag, and he gave them a gift of an amulet each. Grumlok's amplified his strength, Gazbag's magic abilities became far beyond any greenskin's primitive invocations to Mork (or was it Gork?), and both amulets connected them to Malekith's own mind. They both had the focus and longterm planning of the oldest and longest-planning (male) living Elf, and when in doubt could call on Malekith to guide them directly.
Almost as soon as they had returned to the Badlands the two united the Badlands greenskins under the Bloody Sun rulership, and sent them all to attack the Dwarfs in the Eight Peaks...leaving the Empire unable to call on them for help when Tzeentch-aligned Chaos attacked and relying on High Elves instead, drawing away the defenders of Ulthuan...All According To Plan.
As previously mentioned, WAR is a separate continuity from the main Warhammer universe...s. Grum and Gaz had a model (like Grombrindal who has had many silly promo models despite being a canon character), and as a result most likely exist in some form in continuity. They may have just been minor Bosses in the Badlands, or maybe Malekith did experiment with controlling greenskins (in The End Times he manages to convince the avatar of Gork himself to ally with them in two sentences after all, meaning he must be familiar with their politics) but went differently. Its up to those lucky enough to own their model and field it what their ultimate canonicity is.
On The Tabletop
All Badlands greenskins are loyal to them, so all greenskin options other than Forest Goblins are fluffy. Gazbag himself is a Night Goblin while Grumlok is a Black Orc. Ordinary greenskins and Savage Orcs both are commonly found in the Badlands. Players wanting extra WAR flavor can use models from Dark Elves or Warriors Of Chaos models as proxies (which work well for Unit Champions, Musicians, and Standard Bearers) as allied adventurers.