Possession

A hyena possesses Kyle DuFours, Heidi Barrie, Rhonda Kelley and Tor Hauer

Possession was the ability to take over the body of another being, controlling their motor functions and suppressing their consciousness. Trans-possession referred to the act of drawing the possessing spirit out of the body of the possession victim into a new host.

Examples

Demonic possession

Xander possessed after life — Xander Harris, possessed by a demon

Demonic possession was the term used to describe the control over a human form by a demon. Descriptions of demonic possessions included: erased memories or personalities ^[1], convulsions, "fits" of inhuman strength^[2] and fainting as if one were dying ^[3]. Unlike in channeling or other benign forms of possession, the subject had no control over the possessing entity and so it would persist until forced to leave the victim, usually through a form of exorcism.

Some individuals who dabbled in witchcraft or demon worship invoked demons, taking their spirit into themselves by invitation as part of worship.

Vampires implying a form of possession because demons enters in a sired and dead body.
Eyghon the Sleepwalker had the power to possess any dead or unconscious being. If they were dead, they would continue to decompose while he was in their body, only for them collapse and turn into a pile of goo. If this goo came into contact with any dead or unconscious body, then he would take over. In the case of an unconscious host, he would be able to remain in them permanently until their death and had access to some of his powers. If he tried to possess a being such as a vampire, then he had to deal with the demonic spirit inside the vampire's body.
Ovu Mobani could take over the body of any nearby corpse by reanimating it and making it wear the mask his spirit was trapped in, allow him to control their body
The Child of Words, a thaumogenically-made demon, could possess a human host and blend in seamlessly.
The spawn of the Haxil Beast would take over the body of the woman from the very womb they were implanted in.
The demon parasite Talamour possed human bodies that he infiltrate. However, this process kill the victim who is decomposed after a relatively short time.
Ethros Demons enjoyed possessing humans in order to wreck havoc and corrupt their souls. One of these demons was unfortunate enough to take over the body of Ryan Anderson, a soulless child whose evil shocked even the likes of the demon. The demon then made attempts to cause enough raucous in order to convince Angel Investigations to exorcise his spirit out of the boy.
Sluks, due to their incredible thirst, possessed human bodies in order to drain them of their moisture, making the person thrive on more water to keep them self alive. The only way to expel a sluk from the human was to feed them alcohol.
Vampy Cats could possess humans by entering their body orally, and could only expelled by causing the person to vomit.
The Old One Illyria took over the body of Winifred Burkle with the help of of her Qwa'ha Xahn. However, this killed Fred in the process and destroyed her soul as well.

Spiritual Possession

Spiritual possession was when a being who had passed away became "earthbound", such as a ghost, or an astral projection, and began to feed off of the energy of someone who was still a physical being, resulting in noticeable change in behavior.

Some of them drew on the energy of humans who possessed similar emotions.^[4] Usually if a living being had negative emotions they would attract earthbound beings who may feed on them, because these negative emotions were characteristic to most earthbound beings who were usually sad, angry or depressed about their situation, the life they once lived or some other situation (which was usually why they wouldn't move on to the afterlife as they were meant to).

The ghosts of Grace Newman and James Stanley were stuck in a recurring loop where they would reenact the the night of deaths inside the bodies of two people. This eventually ended when Grace took the body the vampire Angel, whose resistance to bullets caused her to survive being shot by James, and allowed her to forgive him so he wouldn't shoot himself.
The spirits of the Lowell House children possessed the body of Graham Miller, causing him to recite the religious chanting they were instilled with.
When Cordelia Chase astral projected during her birthday, she took over Angel's body. Using the limited mobility she possessed, she used him to write a message.
The ghosts of Stefan and an unidentified prima ballerina, two forbidden lovers, possessed Cordelia and Angel, making them have sex in their dressing room.
The First Evil could possess Willow Rosenberg during the performance of a spell.

Animal Possession

File:Hyena transpossession.gif

The zookeeper draws the hyena spirits from the students

Animal Possession was a form of possession where the possessor in question was a wild animal that took on the victim's personal characteristics before corrupting their person. Rupert Giles noted that the Maasai of the Serengeti spoke of animal possession for generations. The Primals were an African cult of animal worshipers who desired to draw the spirits of wild into themselves as a form of achieving enlightenment. Since they preferred only the viciousness of animals, the possession could cause them to slowly lose their sense of humanity, making them become increasingly mean and blood-thirsty as well as increasing their physical attributes.

Body Switching

See Body Switching

Body switching was when two or more beings swapped bodies. This could occur involuntarily and thus constitute as possession.

Other

When initially believing that Sid was an enemy, Willow Rosenberg mentions that on certain cases, inanimate objects of human quality — like puppets, dolls and mannequins — are mystically possessed of consciousness, having acting upon their desire to become human by harvesting organs.
The Bezoar could possess humans via its child. The possessed humans could properly interact with other humans, even though they tended to act monotonous.
Jasmine, a former Powers That Be, took over the body of Cordelia Chase, making her have sex with Connor and conceiving Jasmine herself before giving birth to herself.
Robin Wood was briefly possessed by the Seal of Danthalzar when it was acting up due to the multiple times it was opened.
In the non-canon video game Buffy the Vampire Slayer: Chaos Bleeds, Willow learns a possession spell that makes her enemies attack other enemies. It’s vital for the final boss battle of the level against a vampiric Tara Maclay.

Appearances

References

[1] "Slouching Toward Bethlehem

[2] "Dmaage"

[3] "After Life"

[4] "I Only Have Eyes for You"

[1]

[2]

[3]

[4]