Fiend

Fiends are creatures of the Lower Planes: devils, demons, and other generally evil beings.

Demons
Demons are, like the Abyss from which they come, dreamed into being by Tharizdun, the Chained Oblivion. They are accordingly savage and chaotic, driven by madness and hunger. Their societies are inscrutable: twisting, crumbling, and rebuilding at a truly insane rate.

Devils
Operating out of the Nine Hells, devils are lawful evil creatures of order and tyranny, and they are organized in an extremely rigid hierarchical manner. Their power structure is based on gaining sway over mortals' souls, and their bread-and-butter for gaining influence and power is through contracts. Many devils' whole purpose is to figure out how to gain power and rise in rank by proving themselves or tearing those above them down.

Yugoloths
Yugoloths have not been described in detail in Critical Role media, but in the Monster Manual, 5th Edition, they are greed personified, fighting entirely according to their shameless self-interest, and they hail from Lower Planes other than the Abyss and the Nine Hells.
 * Ultroloth
 * Canoloth: A man-sized, bulldog-shaped creature with armor plating embedded in its hairless skin. Has an exceptionally long tongue extending from a circular, fang-filled mouth.

Other
This list is ordered by descending Challenge Rating.
 * Udaak: Four-armed, ape-like behemoths that wander the plains of Xhorhas. They were brought from the Abyss to Exandria, but they lost their connection to the Abyss when the Divergence happened.
 * Howler: A tall, canine-like, pack-hunting creature native to Pandemonium that uses a vocal sac like a siamang to emit a terrifying, incapacitating howl. Has gray, leathery skin, spiny protrusions along its back, and glowing red eyes. Also can seal its nostrils like a camel to prevent wind-blown sand from getting in.
 * Cambion: Offspring of a fiend (usually a succubus or incubus) and a humanoid. Vox Machina met one who appeared like a very handsome man with deep-red skin, horns, and wings.
 * Night hag: Shapeshifting witches who delight in the corruption of mortals, typically by approaching them in their sleep in ethereal form and filling their dreams with doubts and fears that cause them to turn to evil acts.
 * Succubus and incubus: Shapeshifters who, in ethereal form, approach sleeping mortals and fill their dreams with debauchery, later appearing in the flesh as an attractive humanoid to seduce the mortal into evil thoughts, words, and deeds of their own free will, so that when the incubus/succubus kills the mortal, their soul descends to the Lower Planes. In their true form they have leathery wings, clawed hands, and sharpened teeth. They can charm humanoids and command them.
 * Nightmare: Appears like a horse with red-glowing, intelligent eyes and its hooves, tail, and mane continuously aflame.
 * Hell hound: Like man-sized, fire-breathing dogs.

History
For a history of demons, see here. For a history of devils, see here. The following is the history of fiends other than demons and devils.

Age of Arcanum
Before the Calamity, eight of the Betrayer Gods used the life force of greater fiends to craft sentient weapons for the champions of Ghor Dranas; these are called the Arms of the Betrayers. One of these, Grovelthrash, was forged by Torog with the life force of an ultroloth named Ciria.

Campaign One: Vox Machina
Members of the formative Vox Machina traveled to the Umbra Hills to slay a nightmare that had been spotted in the area; Eskil Ryndarien needed the skull of a nightmare to safely extract a phylactery from Grog Strongjaw's chest. It turned out the nightmare was being summoned by cultists in the local mining guild.

When Vox Machina arrived in Emon for the first time, Syldor Vessar hired them to look for Sir Gregory Fince; when they investigated Fince's home, they found his tortured corpse in the basement, and they discovered and defeated a succubus and several demons.

In Chapter 7, Vox Machina went to the Nine Hells and, in addition to encountering many devils, struck a deal with a cambion named Ipkesh. Later, Percy considered invoking the optional terms of the contract he signed with Ipkesh to bring Vax'ildan back from his service to the Raven Queen.

In, Vox Machina traveled to the Howler's Crag in Pandemonium, and faced a pack of howlers along the way.

Campaign Two: The Mighty Nein
In their first adventure as a party, the Mighty Nein chased a devil to Crooked Stone, an island in the Ustaloch, but the boatman warned them that no one ever goes there because of the witch who lived there a century earlier and who is rumored to occasionally return to a hovel on the island; that witch is a night hag.

The Mighty Nein saw udaaks wandering in the distance when they first entered the wastes of Xhorhas. They saw two more near the city of Asarius, and another in the Barbed Fields.

In Chapter 13, the Mighty Nein investigated a demonic incursion in Asarius and discovered a succubus and incubus in service of some demons who were attempting to build a stable portal from the Abyss beneath the city. The succubus used her charm to turn Caleb Widogast against the party, to terrible effect.

The background for the adventure Merry Spouses of Deastok in the Explorer's Guide to Wildemount notes that incubi and succubi have married into the powerful Truscan family of the Dwendalian Empire over the previous decade.

In the ruins of Aeor, the Mighty Nein encountered a canoloth while they were in the midst of a fight with an Aeorian nullifier; Caleb was able to banish the fiend.

Notable fiends
For a list of notable demons, see here. For a list of notable devils, see here. The following are notable fiends other than demons and devils. Some of the Elder Evils are said to be fiends.
 * Ipkesh, a half-devil cambion in the City of Dis who contracts with Vox Machina to assassinate Utugash
 * The Moon Mistress, a disguised night hag on the islands in Mooren Lake
 * The powerful night hag leader of a coven in the Lotusden Greenwood