So two weeks ago I started talking about a monster type I made called the Edere Vir. Originally envisioned as a part of the World of Darkness, they were humans that preyed on other humans for power and advantage within society.
What could the Edere Vir eat and what could they use those body parts to do?
Feet could help with dexterity or enable them to trip up someone who's running. They could gain memories or general knowledge when they eat brains. They can eat stomachs to induce illness in others or livers to alter someone's alcohol resistance. Hearts could allow them to induce love or affection. Faces to borrow personality as well as appearances. Fat to keep warm. Faces or fingertips to make people who handle their criminal, personnel, or other files more positively inclined towards them. Skin to resist heat, cold, or minor cuts. They can regenerate whatever they've eaten of course, but they can also generally regenerate, but more slowly. Sweat glands to produce someone else's scent. Genitals can be used to induce or diminish lust or fertility.
Another facet them is that, as super-cannibals, they're strongest against members of their own race. Dwarven Edere Vir can bite through and consume Dwarven flesh, muscle, and bone as thought it were well-cooked meat. Same goes for other sapient races, unless you want to put a twist on the Edere Vir of each major race.
Does the condition exist amongst animals? Is is heritable? Cannibal deer and owlbears are really appealing thoughts. Assuming that normal forces of evolution don't apply to the world of D&D--and it's safe to assume they do not--I'd say yes. Otherwise the cannibalistic desire to succeed would only serve an animal's reproductive goal, never override them.