Is there a common, categorical name for creatures that become stronger and more powerful by feeding off living beings' emotions, such as fear, anger, greed?
Attachment spirits. In Native American and Eastern European shamanism, they attach to people either to vicariously live through them, or to whip up the emotions so they can feed off them. This happens in relationships where the argument whips into a frenzy seemingly on its own. (Aside from hormones.) I don't have a citation, I was married to a native shaman who talked about it.
Incubi and succubi are generally represented as being sexual pest spirits that feed off the victim's sexuality, but in the Wikipedia entry for each one, the end result is death. The experience for the victim is pleasure, until they are drained of life force. Sexual pleasure is not quite the same as "emotions", but definitely involves emotion.
In Christianity, demons. "Demons survive by feeding off of emotional vibrations such as anger, fear, joy, excitement and any emotion that vibrates at the frequency needed for the feeding entities. The greater the emotional vibration, the better the feeding session for those feeding. A book by John Klein and Adam Spears titled Devils and Demons and the Return of the Nephilim published in 2005 is a great source for understanding how and why entitles feed off of human energy." http://www.abovetopsecret.com/forum/thread531240/pg2
I know this is part of older esoteric traditions. Qabbala?
I know at least one Christian who strongly believes in this as a reality: http://gnosticwarrior.com/demons-who-feed-on-human-energy.html
- In new age speak, there are Collective Consciousness group-entities that gin up and feed off of any strong "team" related identity -- sports, religion, politics. When you are trying to talk to someone who just won't hear an opposing view, and it almost seems like something is talking through them, and they seem to get a great deal of energy from their group identity, like being a sock puppet for something more powerful. In psychology terms, I'm sure there are specific diagnoses, but still.
I don't think there's a word already existing, but I'm not going to pass up the chance to try and make one. Just combining Latin and Greek roots, a psychophagus would be a "soul eater". There could also be specialized words for each emotion-feeder, such as phobophagus (fear eater), iraphagus (anger/rage eater), etc. Motophagus would be the word for emotion eater, but that's much more questionable than soul eater (psychophagus).
Soul Vampires can be the closest one.
For example, consider the Dementors from Harry Potter, according to the Potter wiki:
Dementors feed upon human happiness, and thus cause depression and despair to anyone near them. They can also consume a person's soul, leaving their victims in a permanent vegetative state, and thus are often referred to as "soul-sucking fiends". They are known to leave a person as an 'empty-shell'.
So, let's see how they are derived from mythology to fit into HP:
They are among the folkloric psychic vampires, so they definitely belong to a type/class of vampires.
So, they can be labelled soul vampires or psychic vampires
A picture from Harry Potter which shows how the soul sucking is done:
Psychic vampires. Here's a Wikipedia entry
There is a popular myth that they feed on people's emotions, especially negative ones. This is also sometimes attributed to people who seem to provoke negative emotions in others on purpose. Basically, one who is considered a narcissist.