I'm currently in the middle of building up a campaign that will revolve around a gnollish army rolling over the land, destroying everything in its path. I don't have the specifics down yet, but they've been united together by a non-gnoll follower of Yeenoghu, to go pillage and slaughter and do other gnollish things. What I'm looking for is the people who would lead this army. So far I have a few ideas, and I'll list them off now.

An annis hag, who supplements her war bands with ogres, trolls, and other brutish creatures. She also has child faces sewn to her skin because annis hags be like that sometimes.

A maurezhi with genius level intellect after consuming an immense number of humanoid corpses. His war bands often have as many ghouls and ghasts as they do gnolls. He enjoys personally infiltrating targets, killing a few people, and destroying the village from the inside after the corpses turn.

Perhaps a nalfeshnee, because I just love the idea of him setting up for a proper meal, silverware and plates and all, while surrounded by slaughter. Perhaps a bit too high cr, though.

As for the BBEG, I'm not quite sure what they'll be. They'll probably more humanoid than any of the others, but I want them to be a bit more interesting than just a regular person who just does tactics good. Really the only thing I know about them is that I'm probably going to give them a flind's flail, to signify the favor of Yeenoghu, as ordinarily only a gnoll can become a flind.

Does anyone have any ideas for other people to lead the gnollish horde? Anything would be appreciated.

A Flind riding a Shoosuva.

Hezrou.

Barlgura 

A wendigo is associated with eating humans but doesn't have a writeup that I know of (could use a yeti). Ghouls eat corpses.

We kind of expect monsters to eat humans. That's kind of what Monsters do. That makes them fearful predators, but not necessarily horrific. Your examples seem to be built to add in an element of horror: children's faces, night of the living dead, the dissonant juxtaposition of eating humans with proper table etiquette.

Have you considering just adding a particular group/tribe/sect of humans into the mix? Perverting the natural order of things adds that level of horror on top of the danger. 

I'd play them up and make them terrifying, and nor remotely sane, but still recognizable as humans.

If you watched Firefly or the movie Serenity, then take a look at the Reavers for the kind of thing I'm thinking about. To me, they're both terrifying and horrific.

I personally am of the belief that the IRL legends of ghouls come from people who were adversely affected by generations of mutations caused by inbreeding coupled with advanced mental illness due to prion diseases contracted through cannibalism. Basically a lot like The Hills Have Eyes. When monsters eat people, it’s a monster eating people. When people eat people, it’s terrifying.

I’m definitely planning on adding non-gnoll cultists, be they crazy bastards who just like slaughtering and eating people, or people driven to madness by torture and magic, to serve as more intelligent commanders of gnoll forces. As the leader himself is a non-gnoll, he recognizes how a more intelligent leader can unite and lead his people to victory.