Codex Gamicus

Twili are the descendants of a tribe of sorcerers called the Dark Interlopers that attempted to seize control of the Sacred Realm after the creation of the Triforce.

They were banished by the Golden Goddesses to a limbo known as the Twilight Realm. Over a long period of time, the Twili have evolved from their original forms to a unique, black and white, gentle-looking race. Two major characters of Twilight Princess, Zant and Midna, are members of this race.

During Zant's quest for kingship, many of the Twili were transformed into shadow beasts, leaving Link to revert those still capable of becoming Twili to their normal selves. In this form, their skin is mostly black with markings on their bodies reminiscent of patterns seen in the Twilight Realm. Their faces, necks, and chests tend to be gray in a marked contrast to the rest of their bodies. The Twili have a stretched appearance, with long limbs, necks, and heads, and great variances in overall height and girth.

It also appears that common Twili are incapable of speaking Hylian and appear to only be able to utter a moan when Link draws near. Both Midna and Zant are unique to their races in that the appearance of their forms (natural or otherwise) are significantly different from other Twili and that they are capable of speaking Hylian.