A gallu has a horned, hairless, wolf-like head, bat wings, pale flesh, hooves, and lion-like tails. They stand eight feet tall and weigh 450 pounds. Gallus can also assume a humanoid form when seeking to instigate war among those with whom they interact.1
Gallus arise from the souls of people who, in life, fomented conflict for the purpose of profit or sadistic pleasure. They are difficult to summon, and are usually only found on the Material Plane after travelling there via a portal.1
Although they often instigate conflict for its own sake while in the Outer Rifts, gallus are crafty and capable of resisting this natural destructive urge on the Material Plane. They prefer to disguise themselves and spend decades building influence until they reach a position with enough power to incite politicians into waging war. Once both sides have suffered enough, only then will the gallu reveal itself to torment the survivors before moving on.1
In the Worldwound, many gallus served as high-ranking officers in demonic armies, each controlling a few marilith generals and reporting to demonic leaders of the Worldwound such as Areelu Vorlesh or Khorramzadeh. A rare few gallus, most often servants of Baphomet, infiltrated the Worldwound's enemies and "advised" them into launching ill-considered attacks against the demons, and sometimes fed their erstwhile allies legitimate intelligence on the demons in the hope of encouraging their violent urges.1
For additional as-yet unincorporated sources about this subject, see the Meta page.