Gnolls—for when you need a generic brutish enemy tribe, but orcs are too weak and ogres are too strong. But I like trying to give the various "evil humanoids" distinct flavor, and I thought of something for the gnolls.

Gnolls, like elves, venerate nature; however, their conception of nature is different. Elves value the idea of nature, fields of wildflowers and dense forest groves teeming with life. Gnolls would look at such places and deem them unnatural, gardens where wildfires and grazing beasts had been withheld. Elves love what nature should be, but gnolls love what nature is.
To an outsider, a gnoll tribe seems composed of bullies and cowards, but this is just the result of how their morality affects their behavior. It is only natural for the strong to take from the weak, for the lion to drive hyenas from their own kills, for the buck with the greatest antlers to mate with all the does he wants, and for those who oppose this order of things to be killed for failing to recognize their shortcomings. As it is in nature, so it is for gnolls. Those with power command those without, they eat their fill and take what they want, and those without power accept these demands.
Unlike many other evil humanoids, gnolls accept all forms of strength. Nature rewards not only the powerful bear, but the clever raven, the hidden serpent, and the cooperative wolf. Whether your power comes from strength, cunning, magic, or even deceit, gnolls will respect it. Gnolls even grudgingly respect those with political power, if only for the audacity in gaining power by acting like they have power.
But the weak in gnollish society don't just roll over and play dead. All things in nature grow, or are destroyed. Gnolls seek to increase their personal power one way or another. Some train their brawn, some seek magical prowess, some search for powerful groups to attach to or convince their fellow gnolls that anyone following them will gain power (or other things gnolls want—they don't only want power, any more than a dwarf only wants gold or an elf only wants to hang out in sunlit groves).

Some gnolls follow more powerful beings. Giants, dragons, and warlords are common, but very few follow gods. Gods almost never use their power to influence the world, and to a gnoll, power that is almost unused is almost not power at all. But there is one being who many gnolls revere like a god, and she is Yeenoghu.
In life, Yeenoghu was a mighty arcanist, warrior, and strategist. She gathered a tribe of gnolls like few others, and invited anyone else who wanted to bask in her power to join with. But never did Yeenoghu try to establish something that would outlive her power, like an empire or a cult of personality; what she did would last her lifetime and then fall apart for others to empower themselves with, like the trunk of a tree which grants food and shelter to all the woodland creatures elves claim to love when they clear those logs away.
Upon Yeenoghu's death, she was condemned to the Abyss. Yeenoghu was infuriated; had she not lived an honorable life, taking nothing she did not deserve, building nothing she did not need, and following the laws of nature? Shouldn't she be granted the glory of the heavens, like any other mortal hero? These were the questions Yeenoghu asked herself as she was dragged screaming through the layers of the Abyss.
Yeenoghu had suffered in death, but she was still strong. As the demons which first found her began their torment, Yeenoghu slew them. During her first days in the Abyss, she discovered that eating demonflesh would bolster her power further, so she began to actively hunt demons. She was a thorn in the Abyss's side until some demon knights helped her carve out a territory of her own, with her own demons to form a new tribe. Even then, she regularly raided neighboring layers of the Abyss so frequently that Asmodeus considered making her an ally, before another demon lord just gave her the secrets of extending her influence to the mortal realm.
Thus was the meteoric rise of Yeenoghu, the newest demon lord in the Abyss. Today, Yeenoghu regularly interacts with her followers in the mortal plane, striking down their enemies and any who seek to disrupt the natural order. Meanwhile, she seeks for some means of achieving her true goal. Some demons want power in the Lower Planes, some want to spread evil across the Material Plane, but Yeenoghu reaches for the heavens themselves—not to conquer, slaughter, or pillage, but because she feels it is her right to rest her laurels in those hallowed halls, enjoying the peace earned through a life red in tooth and claw. Wouldn't you think such a reward for her virtue would only be natural?