So this is how powerful demons are. No wonder why they're taller than imps. It's actually confirmed that sinners/ Overlords are powerful than imps and Hellhounds. If you think about it, imps and Hellhounds are the only Hell species who don't have demon forms.
For instance:
1. Lucifer Magne: Lucifer is basically the creator of Hell, but it's actually Vivianne. It makes sense for him to be powerful and he is a fallen angel.
2. Charlie and Lilith: Lucifer's family are the second most powerful Hell species in Hell. We seen Charlie in her demon form, but that's only when she's extremely mad. We're going have to wait until we officially see Lilith since we know that Lucifer will be in the series.
3. The Seven Deadly Sins: The Seven Deadly Sins, like Pride, Wrath, Gluttony, Lust, Envy, Greed, and Sloth. Lucifer is Pride, Satan is Wrath, Bee-lzebub is Gluttony, Asmodeus is Lust, Charlie's ex boyfriend's dad is Envy, Mammon is Greed, and Belphegor is Sloth.
4. Arts Goetia: Arts Goetia, such as Stolas, Octavia, Stella, Paimon, and others are Hellborn royals, which they are owls or bird species. Like for example, Stolas is a owl and Stella is a swan. So they can be pigeons, hummingbird, woodpecker, rooster, chicken, and ducks.
5. Overlords: Overlords such as Alastor, Valentino, Vox and Velvette. They're more powerful than regular sinners.
6. Sinners, such as Mrs Mayberry, Husk, Vaggie, Angel Dust and his family minus his twin sister, Molly, Niffty, and other. They're demons who used to be humans. They're appearances are based on what they looked like when they were alive and how they died. Like Angel's family are spiders due to their "webbed" crimes, so they take a form as spiders. Sir Pentious possibly died from a snake or serpent bite because they can kill you, Mrs Mayberry committed suicide, Alastor was shot, the hunter mistook him for a deer/ a dog incident since it's confirmed that he hates dogs because it had something to do with his death, so he takes a form as a deer. As for Vaggie, Husk, Cherri, Sir Pentious, and Nifty, we don't know about except for what year they died. It's confirmed that Vaggie is 20 years old, and she died in 2014, which was way after the others died. Cherri's hair is based on a bomb. And when they enter Hell, they change their name to whatever they want. Like Angel's real name before he died was Anthony, Vaggie's real name is Vagatha, Alastor kept his name the same since he hates nicknames.
7. Hellborns: Hellborns like Succubus, Succubi, and others.
8. Imps and Hellhounds: You already know about them.