Fog Feet

Perhaps a combination of Partial Transformation and Super Smoke or a general characteristic of beings that aren't of this earth, including but not limited to genies and ghosts. Characters with fog-feet operate as if they have a full anatomy, but some portion of their lower body is invisible or insubstantial.

Compare and contrast Invisible Anatomy. For the trope where feet are blurred due to speed, see Wheel O Feet.

General

 * This is a common representation of genies. While the upper torso appears solid, the legs ends in a wisp of smoke, generally connected to the lamp, bottle or whatever genie artefact they're supposedly coming from.

Anime & Manga

 * Sayo from Mahou Sensei Negima even manages to trip over her nonexistant legs.

Comic Books

 * Secret from the comic book version of Young Justice.
 * Veil from Avengers Academy.

Films -- Animation

 * Aladdin: Genie had a tail of the same shape as that of an archetypical genie most of the time.
 * Hades on Hercules, whose robes dissolve into wisps of smoke.

Films -- Live-Action

 * The library ghost in Ghostbusters.

Live-Action TV

 * Charmed has demons made of smoke; they wear cloaks and don't have feet, but still hover and such.

Myths & Religion

 * In Japanese mythology, Ghosts don't have legs.

Tabletop Games

 * Dungeons and Dragons
 * Angels in 4th Edition D&D.
 * Water and Air archons have this too.
 * Pathfinder: Wendigos have legs, just not feet.

Video Games

 * Touhou has Mima, Konngara, and Soga no Tojiko (funnily enough, she has two of them).
 * In Sonic Adventure 2, a chao that plays with bats will eventually have its legs disappear; it'll hover above the ground instead.
 * The Firelord hero from Warcraft 3 has a humanoid torso, but moves along on a tail made of magma.
 * Ghosts in Puzzle Pirates are wisps of smoke from the waist down, but act as if they had normal legs.
 * Zettai Hero Kaizou Keikaku has an item that replaces the main character's legs with a cloud.
 * Tornadus, Thundurus, and Landorus from Pokémon Black and White all have clouds for lower torsos.
 * In Baldurs Gate II, the genie model (used for Djinn, dao and, less apropriately in terms of looks, efreet) has both a legged and a "tapering down to a point as if emerging from a bottle or lamp" version.

Web Comics

 * In Sins, Lust tends to get stuck with this a lot.
 * Ghosts in minus have fog-feet; but they can fly, so they don't really need feet anyway.
 * Kernelsprites in Homestuck, which is appropriate since most are prototyped with and take on the memories and form of dead people close to the player. Curiously, though, actual dead people and ghosts don't follow this trope.

Western Animation

 * Danny Phantom: Danny had a ghost-like tail some of the time.
 * The Faceless One from He-Man and The Masters of The Universe was always portrayed, both in animation and comic books, as a ghostly figure with mystic smoke around his legs. When he finally received an action figure that had no representation of the smoke, many fans were displeased.
 * The Venture Brothers: The villain known as The Phantom Limb has a very dangerous version of this trope.
 * Wakfu
 * McDeek the Genie, in his true form, has a long trail of purple smoke instead of legs.
 * Rushu, lords of the Shushus, is a special case: his lower body is replaced by searing flames.