It is an extremely difficult task to categorize members of demonknd due to the sheer number of anatomical possibilities and circumstances of their creation. Most demons hardly see the need for such categorization attempts in the first place though they do recognize six general morphs that are commonly seen amongst their kind. These morphs are known as namuizuu (meaning "flesh-shapes") are six broad anatomical categories that describe a loose collection of common traits that demons within that category share.
The other factor in demon classification is which protean the demon is descendant from. A grouping of demons descended from the same protean is known as a brood. Brood has a huge influence on a demon's nature: the demons of Ur-Eresh's brood are draconic creatures of destruction while Ur-Ursagaru's brood are often furred beasts with great tracking senses for instance. Most demons do not instinctively know of each other's broods unless they are from the same brood, in which case they seem to innately know such connection.
Thus a demon's classification is usually a combination of Brood and Morph, with Brood determining much of the demon's nature basic traits and Form determining how those traits are expressed. This classification method isn't something demons formally recognize, rather it's basic understanding that has been reached over time.
Ish (ISSH) – Demons of this flesh-shape are distinguished by their gelatinous and sometimes amorphous forms. Any organs and bones the demon may have are suspended within the viscous fluids of the demon’s insides. Their proportions may sometimes appear somewhat cartoony with bulging eyes and over-sized mouths. Their fluid nature allows them them to take on a slime-mold like form that allows them to fit into small spaces.
Eza (EE-ZA) – Resembling spectral wraiths, the demons of this flesh-shape have ghostly translucent anatomy. In place of hair or fur, there are trailing wisps of vapor. Any bones the creature may have emit a faint glow that shines through its misty form. At the expense of some of their plasm, they can diffuse into their surroundings, melding into the environment.
Suu (SOO) – Demons of this flesh-shape resemble somewhat abstract clay sculptures. Though they still resemble beasts, they often have surrealistic proportions and may be covered in etching-like markings. Some portions of their body may have geometric shapes such as hexagonal eyes or triangular scales. They retain blessings longer than most demons and can safely absorb curses without suffering their effects.
The most corpse-like of all flesh-shapes, Ruu (ROO) demons possess sunken eyes, exposed bones, and emaciated forms. Their skeletal forms are often wrapped in strange jelly-like webbing or strips of flesh that resemble bandages, making them appear mummified. Due to their strange anatomy, they can assume a death-like state allowing them to go without nourishment for long periods of time and slow-down the spread of poison.
Tza (TEE-ZA) demons defined by the presence of exaggerated musculature and hardy constitution. Demons of this type often possess over-sized claws, thick carapaces, elongated fangs, and other powerful natural weaponry. When sufficiently stressed or excited, Tza demons are known to transform into an atavistic form more closely resembling the Protean they derived from.
Ezu (E-ZU) demons ppear to be strange hybrids of plant and beast. Toothy maws are nestled within curled petals while thorny vines slither take the place of fingers. While all demons possess some plant-like traits, the anatomy of these specific ones is defined by that nature. They are capable of spawning seeds that blossom into cursed/blessed flowers.