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. If your Humongous Mecha trades anthropomorphic silhouette for mobility see Hover Mecha or Hover Skates.
- Sayo from Negima! Magister Negi Magi even manages to trip over her nonexistent legs.
- Caesar Clown from One Piece constantly uses the power of his Gas Gas Fruit to transform his lower body into gas, which gives him an appearance reminiscent of a traditional Shinigami. When forced into his normal state, he ends up looking rather ridiculous.
- Dungeon Keeper Ami: The ghost fish that Mercury makes is a variant:
... the front half of a fish. Everything behind the main fins dissolved into a trail of indistinct mist.
- Aladdin: Genie has a tail of the same shape as that of an archetypical genie most of the time. When his connection to the lamp is broken at the end of the film by Aladdin's final wish, the tail is broken off of the lamp and he grows a pair of legs... though the legs are seen at least once before that.
- Hades on Hercules, whose robes dissolve into wisps of smoke.
- My Little Pony: Equestria Girls Legend of Everfree: The Gaea Everfree in Timber Spruce's story doesn't have legs, but an amorphous trunk-like lower body that seems to flow from the ground, leaving a trail of gem dust.
- The library ghost in Ghostbusters doesn't have legs — just the trailing, ragged hem of her dress.
- La Saga du Prêtre Jean: In book 2 L'il du Sphinx (The Eye of the Sphinx), hostile desert spirits attacking a caravan look like humans above the waist, but have nothing but a whirlwind below.
- Charmed has demons made of smoke; they wear cloaks and don't have feet, but still hover and such.
- In Quantum Leap, Sam leaps into a Vietnam vet who's lost both his legs. There's a mean orderly in the episode, and at one point when there's nobody else about, Sam gets out of the wheelchair, walks over to the orderly, and slugs him. Since the orderly can only see Sam as the man he leaped into, it looks to him like Sam is floating on air.
- This is a common representation of genies. While the upper torso appears solid, the legs end in a wisp of smoke, generally connected to the lamp, bottle, or whatever genie artifact they're supposedly coming from.
- In Japanese Mythology, ghosts are typically depicted as legless.
- Cartoon depictions of ghosts in Western culture often portray them as legless, but there's no real cultural evidence that ghosts are thought to be this way... it seems to simply be a visual shorthand to imply their otherworldly nature.
- Dungeons & Dragons:
- Angels in Dungeons & Dragons 4th Edition have bodies made of what look like strands of light that trail off into a tail.
- In Sonic Adventure 2, a chao that plays with bats will eventually have its legs disappear; it'll hover above the ground instead.
- Sonic the Hedgehog (2006),: Mephiles has no feet in his crystalline form and hovers just above the ground with a dark-purple mist-like effect. Interestingly enough, his legs are still present, but everything from his knees down is replaced with crystalline stumps.
- Warcraft III: The Firelord hero has a humanoid torso, but moves along on a tail made of magma. The Firelord design extends to Fire, Air, and Water Elementals in World of Warcraft, with a special mention going to Voidwalker demons. The most impressive example of this trope would be Ragnaros, who is the Fire Lord and bigger than ten lesser elementals standing on each other.
- Onmyōji: Kisei always levitates in a sitting position because of this. However, he's shown to be able to occasionally stand up in midair.
- Puzzle Pirates: Ghosts are wisps of smoke from the waist down, but act as if they had normal legs.
- Zettai Hero Project has an item that replaces the main character's legs with a cloud.
- In Baldur's 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.
- Panel de Pon: Elias has normal feet, but her lower body transforms into a cyclone during her attacking animation. One of her official artworks◊ evokes this, too.
- Catacomb Fantasy Trilogy: In Catacomb Abyss, spectres (if that's really what they're called) are bat-featured vaguely humanoid insubstantial-looking creatures that taper down to a point and float just above the ground.
- Shin Megami Tensei: This is a common trait of spectral or semi-incorporeal demons:
- The Native American cloud people Shiwanna, whose body solidifies and dissipates into purple wisps.
- The Aztec Tezcatlipoca, who has legs and the hint of feet, but as he strides forward those feet just... fade into a vapor.
- The entire lower body is replaced by a cloud for Mega Man 7's Cloud Man and his Mega Man Battle Network counterpart.
- Yu and Rei, the ghost twins from the Puyo Puyo series, have fog feet. Which creates an In-Universe Fridge Logic situation for other characters when Yu starts dancing.
- In EarthBound Beginnings and EarthBound, defeated party members appear as floating ghosts with tails instead of legs. Mother 3 breaks this tradition by having defeated party members remain alive and corporeal, but too weak to fight.
- Jimmy and the Pulsating Mass For ghosts in general:
- The Information Guy's ghost, which appears after the Revolting Blob fight when Buck kills him.
- Rad Ghosts don't have feet, their bottom half just is a thin line that curls.
- The Djinn of Dungeon Crawl don't have legs; they can't wear boots, but they are permanently flying.
- The deadbeats from Mystery Skulls Animated are simple purple ghosts who taper off to a lightly transparent point in lieu of having legs or feet.
- Ghosts in minus. have fog-feet; but they can fly, so they don't really need feet anyway.
- Sprites 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. Which doesn't prevent John from calling it a "ghost butt".
- Ghosts in Our Little Adventure are pictured with a wisp of smoke instead of legs.
- The Order of the Stick:
- A djinni in the flashback of Celia's job interview has a wisp of smoke for legs.
- The visual representation of the Wind Walk spell is for the characters' legs to be replaced with vapor trails.
- Unlike other gods, Hel seems to be lacking legs, her lower body ending in a wisp like a ghost's shroud. it's especially obvious when she's flying.
- The Lilitu from Underling, primitive nature demons from ancient times, have the form of female imps with a trail of smoke instead of legs. Including main character Lilith, although she can take a more humanoid shape.
- Ghosts in Paranatural have this. Played for Laughs when Dr. Zarei is treating a ghost patient:
Ghost: Doc, I—I can't feel my legs.
Dr. Zarei: You don't have legs.
Dr. Zarei: ...This is not a new development. You lost them when you died.
Dr. Zarei: That's not new either.
- Ghosts in Champions of Far'aus have this in their "distinguishable" form.
- The Blue Ghosts in The Redacverse are drawn with a stubby, legless lower body and float above the ground.
- Danny Phantom: Many ghosts, including Danny himself, can form a ghost-like tail when they're flying. Some ghosts have one permanently, such as Clockwork and Desiree (justified in the latter's case, since she's a genie ghost).
- The Faceless One from 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.
- 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: he can replace his lower body by searing flames if he chooses so.
- In season 2 episode 22, both Ecaflip spirits Mélo and Sho-Bubu lack legs, having instead ectoplasmic "tails" linking them to their respective cursed rings.
- SpongeBob SquarePants: The Flying Dutchman has no feet, just what he refers to his as "my ghostly tail". Sometimes he wears a sock on it.
- Casper's Scare School: All ghosts have genie-like tails most of the time. However Casper is shown to be able to grow a pair of legs when needed, so presumably other ghosts can do this too.
- Teen Titans Go!!: In the Halloween special, Scary Figure Dance, the Titan ghosts have ghostly tails instead of two feet.
- Shadow Raiders brings us Lamprey, a null-matter being with no feet or legs, her lower half being a fish-like tail, who just hovers around.
- My Little Pony: Friendship is Magic: In "A Hearth's Warming Tail", the Spirit of Hearth's Warming Yet to Come, portrayed by Princess Luna. The bottom of her cloak seems to be ghostly and immaterial, with wisps of smoke curling into themselves constantly, mixing the same colors as Luna's usual mane.