One-eyed ghoul

A one-eyed ghoul (隻眼の喰種 Sekigan no Gūru) is a human-ghoul-hybrid. Most ghouls treat their existence as an urban legend or have never heard of their existence at all.

Characteristics
Hybrids are labeled One-Eyed Ghouls because they only develop one kakugan, in contrast to normal Ghouls. In all other ways, they appear to have no obvious differences from an ordinary Ghoul. Some individuals are noted to have a unique scent that combines traits of a human and a Ghoul, identifying them as different and often considered "enticing" by certain ghouls; Ken Kaneki has been shown to both smell like a Ghoul by some individuals whereas some cannot tell the difference, and Eto has been shown to smell like a human so far. It is unknown whether all hybrids have this trait. Their flesh and blood is also noted to be particularly delicious, making them a potential target for cannibalism.

The one-eyed ghouls Ken Kaneki and Eto are not detected by CCG's Rc Scan Gates. However, Eto claims that this is unrelated to their half-ghoul nature; rather, they are both related to ghouls related to V, and the Rc scan gates are set up such that they don't detect ghouls affiliated with V.

Rumor states that hybrids are more powerful than ordinary Ghouls due to hybrid vigor, though it has not been further elaborated on. At least three specimens (Eto, Kaneki, and Takizawa) have demonstrated radical evolution of their abilities and developed a Kakuja within a short period of time. Physically, their bodies are identical to that of a Ghoul with the same resistance to harm and ability to use a Kagune. Like normal Ghouls, they are only able to consume meat of human or Ghoul origin, as well as coffee and water; Eto has been shown consuming human food, but it is unknown if she can eat it without any adverse effects at this time. As such, they are inherently a cannibalistic organism. Hybrids come into existence through two methods, either normally born as a result of the union between a male Ghoul and a female human, or through transplanting a Kakuhou into a human host.

Pregnancy
Naturally-born hybrids are extremely rare. The chance of pregnancy is low, and even if the mother becomes pregnant, the child normally dies during pregnancy: If the mother is a human, the child will not get necessary nutrients and will starve to death unless the mother were to engage in cannibalism. If the mother is a ghoul, the mother's body will mistake the child for food and reabsorb it. Eto is the only known naturally-born one eyed ghoul.

Kakuhou Transplant
The primary method of creating One-Eyed Ghouls is through transplanting a Kakuhou into a human recipient, an exceptionally high-risk procedure. In the rare event that the operation is successful, the human patient develops into an artificial hybrid with all the abilities of a normal Ghoul. This procedure was pioneered by Dr. Akihiro Kanou, with the first successful patient believed to be Ken Kaneki.

Quinx Squad
After examining Dr. Kanou's surgery on Ken Kaneki, the CCG created an experimental squad consisting of humans with ghoul capabilities through implementing a kakuhou covered by quinque steel layers. While the Quinx Squad's members aren't considered as normal half-ghouls due to their lower RC cell levels and the ability to eat human food still, they are made from the concepts of Dr. Kanou's half-ghoul experiments.

Natural

 * Eto
 * One-Eyed King (Rumored)
 * The First One-Eyed Ghoul (Likely)

Artificial
Until now, Kanou has created two series of artificial ghouls: The Eater-series artificial ghouls created from "Binge Eater" Rize Kamishiro's kakuhou, and the Owl-series artificial ghouls produced from "Owl" Yoshimura's kakuhou.

Rize-based

 * Ken Kaneki/Haise Sasaki (K-Eater)
 * Kurona Yasuhisa (B-Eater)
 * Nashiro Yasuhisa (W-Eater)

Yoshimura-based

 * Seidou Takizawa (T-Owl)

Unknown base

 * Koutarou Amon

Trivia

 * Itori and Uta share rumors of another One-Eyed Ghoul with Kaneki, advising him to not seek out this being because of the danger involved. It is likely they were referring to the One-Eyed Owl.
 * Eto is the only confirmed case of a natural-born half-Ghoul.
 * Fujishige Iba once mentioned to his subordinates that there was a scientist that wanted to experiment with hybridization, but his research was deemed unethical and shut down.
 * One-Eyed Ghouls are stated to be more powerful as a result of Heterosis, also called "Hybrid Vigor". This phenomena involves the offspring of two different breeds or species inheriting superior traits from its parents, and is frequently used in agriculture and animal husbandry.
 * Akihiro Kanou refers to the failures from his experiments as Floppies, though it unclear what difference there may be between a Floppy and successful artificial One-Eyed Ghouls. However, it could be that they are not as strong as he hoped. This was shown when he compared the Yasuhisa twin to Kaneki he mentioned that they weren't as strong as him thus they are failures, thus floppies.
 * According to Eto, about a hundred years prior to the current timeline and events in the manga series, there existed a One-Eyed Ghoul who was so vastly powerful that humanity had to come together in order to defeat it. It's eventual defeat is said to have led to the creation of the CCG (and perhaps various other ghoul prosecuting agencies around the TG world). Due to how strong they were said to have been, it's very likely that this individual was a natural born hybrid, like Eto herself.