There are numerous undead, phantasmal, fantastical, or otherwise supernatural entities that inhabit Sonic's world. More often then not these beings frequent areas that have well-deserved reputations for being haunted.
Ghosts known as Hyudoro inhabit the pyramid in which Act 2 of Sandopolis takes place. They are initially trapped in a capsule, but unwittingly freed by Sonic or Tails. They hover around the top of the screen, becoming more menacing if the stage is allowed to become darker. Some time after the level becomes completely dark, they begin to charge the player and can inflict damage. Although they can be beaten with any attack, they will reappear shortly afterward. Knuckles has to deal with these belligerent spirits from the beginning of the act.
Chaos, described by Doctor Eggman as "the God of Destruction" is the water-based guardian of chao who was imprisoned in the Master Emerald after destroying the Knuckles Tribe 4,000 years ago. He is later freed when Dr. Eggman shatters the Master Emerald, intending to use Chaos to take over Station Square. Chaos is seemingly immortal and becomes stronger by consuming Chaos Emeralds. His final form, Perfect Chaos, can summon tidal waves, twisters, and is only reigned in by the efforts of Super Sonic. Additionally, according to the codex of Sonic Chronicles: The Dark Brotherhood, Perfect Chaos caused an energy backlash that propelled Angel Island into the sky, where it has mostly remained since.
With the exception of flashbacks and in Super Sonic's story, Tikal appears as a glowing ball of pink light. In this form, she can send characters into the distant past and back, and seems to have a heightened awareness of the location of Master Emerald shards, though this may be a trait that members of the Knuckles Tribe possess.
The creepy prisoners of the interior of Red Mountain are composed of nothing except for a pair of yellow eyes and thin strips of material that collectively take the shape of a roughly human form. They mostly function as scenery, although in Mission 30 of Sonic Adventure DX, one has escaped from its cell and Sonic must track him down.
Finally, in the Halloween Party download available for the original Sonic Adventure from October 31st, 1999 to December 31st, pumpkin-headed ghosts donning hats, cloaks and lanterns haunt Twinkle Park. They will not harm the player and convey "Trick or Treat!" when approached.
Ghosts reappear as proper enemies, haunting desolate mountainous regions and pyramid ruins. Tiny Boos either wander about or hide in scenery, and spook the player when approached. Normal sized Boos either home in on the player and restrain them temporarily, or bump into the player and cause damage. Larger Boom Boos cause damage on contact and take three attacks to vanquish.
The gigantic King Boom Boo acts as a boss for Knuckles. Unlike other ghosts, who appear at worst only slightly hostile, King Boom Boo viciously attacks Knuckles, attempting to bite him and burn him with projectiles and streams of blue fire. Luckily, King Boom Boo can be damaged when exposed to sunlight.
Cloaked Pumpkin-headed ghosts of several sizes haunt Hang Castle and Mystic Mansion. Although they do cause damage when touched, they don't generally won't go out of their way to hurt you, preferring to mill about a small area. A slightly more antagonistic variety tends to suddenly appear as your team approaches. Either type will disappear if attacked, though they'll reappear shortly.
Mystic Mansion also houses a harmless skeleton who likes to peep around corners. It always manages to slip behind a wall and disappear as your team gets near.
Transparent blue ghosts slowly rise from vents and towers around Eggman's Cryptic Castle. They cause damage on contact. Cryptic Castle also contains a skeletal beast who pursues Shadow as he rails towards the Goal Ring.
The ancient genie Angelus the Gatekeeper protects the treasure of Babylon Garden in the Babylon Guardian course. Groaning ghouls drag your racer towards the abyss in Digital Dimension. However, given the virtual reality-like nature of these courses, the supernatural entities inhabiting these tracks may be nothing but artificial projections.
Iblis and Mephiles the Dark are the split characteristics of the omnipotent sun god Solaris. Iblis is an immortal and destructive fire spirit who renders the Earth of the future almost entirely uninhabitable. Though mindless, Iblis has many dangerous powers, including shifting between at least three known physical forms, controlling twisters and pulsating globes of flame, and creating various fire beasts. Aside from being a master of manipulation, Mephiles is able to closely duplicate a being's form by possessing its shadow, summons his own minions, creates countless clones of himself (with the assistance of two Chaos Emeralds), and casts Chaos Spear-like beams, which he uses to kill Sonic. When Iblis and Mephiles reform into one, Solaris envelopes the world into his own distorted dimension, with the intention of destroying all of existence.
The vortexes that appear in End of the World appear to have some sort of an awareness, as their slit pupils follow the player.
Genies, known as djinn in Sonic and the Secret Rings, abound in the world of the Arabian Nights. Shahra and the evil Erazor Djinn can grant wishes when their corresponding object is rubbed (for Shahra her ring, and for Erazor Djinn his lamp). Erazor Djinn's minions are numerous summoned djinn who have many distinct appearances and abilities, including disappearing and reappearing, breathing fire, and temporarily turning Sonic into stone. Also controlled by Erazor is the Ifrit Golem, a mechanical djinn covered in flame and only vulnerable when exposed to the World Ring of Water.
King Solomon appears as an animated talking skull when you first discover him, his bones scattered by the Forty Thieves who have been resurrected by Erazor Djinn. Though initially irritable, he'll gratefully tell your where to find Erazor and how to seal him once Sonic recovers the King's bones and defeats the Forty Thieves.
Other entities include Uhu the Wind Genie, a speedy sphere of blue light who challenges Sonic to races, Pri Ma Djinn and Kri Ma Djinn, two djinns who appear in the game's Party mode, and flying chests, pots, and carpets.
Many of the enemies populating Haunted Ship have phantasmal qualities; One even disappears and reappears from time to time. However, the nuts and bolts that burst forth whenever the player defeats them suggests that they are, in fact, robots built by Doctor Eggman or Eggman Nega.
Dark Gaia is described as a "hyper-energy organism" by Professor Pickle. This supernatural behemoth slumbers at the core of the planet for millions of years at a time, along with its better half, Light Gaia (Chip). Woken up too early by Dr. Robotnik, Dark Gaia's world-sized form collapses into innumerable Dark Gaia minions at the start of the game. These creatures pose both physical and psychological threats to Sonic and the humans; able to possess people and generate negative emotions as well as fighting Sonic the Werehog.
The mysterious Argus is mentioned in Sonic Chronicles: The Dark Brotherhood, as the name of a hypothetical entity the Precursors held responsible for the operation of the Twilight Cage. If he exists, Argus is responsible for the Argus Events, imprisoning the Precursors, Kron, N'rrgal, Zoah, Voxai, and Nocturnus within the prison dimension.
When Sonic arrives in the Grand Kingdom, the tyrannical King Arthur is subjugating the land with the assistance of various demons summoned from the underworld. Dissolving into a black mist after being defeated by Sonic at Faraway Avalon, Merlina the Wizard reveals that King Arthur was an illusion created by her grandfather, Merlin, in order to unite the Grand Kingdom.
Caliburn, the sword that Sonic wields in the game, is capable of speech and has a distinctly chivalrous but snide personality. Fairies, winged globes of light, take the place of some of the standard objects in Sonic games. For example, yellow ones bestow rings when touched, blue ones act as springs and dash panels, and red ones fill the Soul Gauge.