Legacies - and built a large fantasy world with rules and supernatural beings that have special powers for one reason or another. Since this became such a huge universe, there are a lot of different supernatural species presented to the audience throughout the episodes, even more so recently in Legacies.
Some are less iconic and memorable than others, and there are those that take the spotlight and feature more prominently across all three shows because of their strength, power, and because of how complex and interesting they are.
Spirits
When a human or a supernatural being es away they can become spirits that either get sucked into hell or reside in peace, where they can watch over their loved ones. Jo described it as a dream: "I used to watch all of you. Like in a dream where everything is warm and happy. I think I was at peace. So that's where I'll be. Waiting for you."
These are some of the most benign supernatural beings, as they don't really possess much power unless they had supernatural abilities prior to their death - like for example the Bennett witches who still had their powers when summoned.
Travelers
During the period of ancient Greece, a powerful witch Qetsiyah created the Immortality Spell, which caused the witch community to divide. The traditional witches believed that natural law must be respected. The witches cursed the Travelers with a spell, barring them from using traditional magic - breaking their connection to Nature.
Due to their magic not needing a balance, they are quite powerful, at some point planning a spell that would make vampires extinct. Before becoming a vampire, Katherine was also a Traveler, which enables her to jump into Elena's body when she's about to die.
Doppelgängers
Doppelgängers, or Shadow Selves, were created by Nature to keep the balance. When Silas and Amara became immortal, they violated the natural law that says all living things must die, and that's why the doppelgängers were created - so there could be a version of them that could die.
There were two lines of doppelgängers - the Salvatore and Petrova doppelgängers. Some TVD doppelgängers are more evil than others, but they are all essentially human - while being a supernatural occurrence, they contain no magic. When Kai attempted to siphon magic from Stefan, unknowing that he had taken the cure, he couldn't siphon magic, despite Stefan still being a doppelgänger.
Supernatural Hunters
Hunters are humans with supernatural abilities that enable them to hunt and kill vampires. They are essentially human, but they're still considered supernatural and go to the Other Side when they die. The first vampire hunters that existed were The Brotherhood of the Five, created by a Bennett witch.
The hunters have physical abilities that exceed humans, like strength, speed, and endurance. They are marked with a tattoo, called the Hunter's Mark. The most notable hunter was Jeremy, who activated his Mark after another hunter died. His thirst to kill vampires got so bad at some point, that he attempted to kill his own sister. Even though they're strong, they're not as strong as vampires, and only their skill makes them capable of killing one.
Sirens
Sirens were granted immortality, but it was dependent on the feeding of human flesh for them to remain young and beautiful. They made a deal with Arcadius to harvest the souls of the wicked for him, and in return, they'd be immortal.
They have great telepathic abilities and are able to control minds, manipulate memories, and even imitate voices, as seen when Sybil imitates Elena's voice to lure in Damon. This is an interesting nod to the siren myth, which says sirens lured sailors to shipwrecks with their singing. Their powers seem to stem from Sybil and Selene being psychics prior to making the deal. Sybil and Selene are the only known sirens to exist in the TVD universe.
Werewolves
These are shapeshifting beings, who transform into large wolves against their will every time there is a full moon. The lycanthropy gene is ed down and lies dormant until it's triggered by the act of killing someone (even in self-defense or on accident).
They possess superhuman physical strength and their bite is lethal to vampires, which makes them their biggest enemy, especially because they are wired to kill vampires. But their physical strength does not sur that of a vampire. The most notable werewolf in the show was Tyler Lockwood, prior to becoming a hybrid. It is also shown that werewolves are very loyal to their packs, unlike vampires, and will often turn against their friends (like Tyler turned against Caroline) to protect their own.
Vampires
Vampires are one of the most common species featured in The Vampire Diaries. This show stuck with the somewhat classic version of vampires who burn in the sun and depend on a blood diet. However, in many ways, the TVD vampires are different from conventional ones. The process of turning someone into a vampire is relatively simple. They must have vampire blood in their system, they must die, and after reawakening, they must feed on human blood.
A diet of blood doesn't only satisfy their hunger but also enhances their supernatural abilities. When enjoying a diet of human blood, they can manipulate humans, which is called compulsion. Examples of vampires from the show include the Salvatore brothers, Caroline Forbes, and later on Elena as well. They are undoubtedly the stars of the show, even though the Originals are stronger and can compel other vampires. Even most witches are often seen as stronger and can take them down.
Witches
Witches are born as humans and remain human until they tap into their magic. They are born with their power, which is ed down from generation to generation. Their magic stems from the fact that everything in nature must be in balance, so if a witch for example brings someone back from the dead, someone else must die instead. Bonnie did bring Jeremy back, but she sacrificed herself in return, resulting in one of the saddest deaths in The Vampire Diaries.
They live and practice magic in covens. It is interesting that they often think they're above mixing with werewolves or vampires and are on neutral to negative with both, even though ironically witches created both species - Esther Mikaelson created vampires and a witch named The Hollow created werewolves. Among the most powerful witches on the show were Qetsiyah, Silas, and Esther Mikaelson.
Original Vampires
The Originals are a group of first-generation vampires and are the most powerful vampires to exist as well the oldest, fastest, and strongest. The term is used to refer to the original family or the Mikaelson family. There are two other types mentioned - the Enhanced original vampire (Alaric) and the Upgraded original vampire (Lucien Castle and Marcel Gerard).
The original vampires were created because their parents wanted them to become immortal and superior to werewolves. They were not converted but became vampires because of a spell. They can also only be killed by a white oak stake, which gives them their immortality. The price they paid for it was also the hunger for blood, weakness to the sun, and inability to enter houses without being invited in, which is interesting because, in that regard, they don't differ from a regular vampire.
Hybrids
Hybrids are made by cross-breeding or mixing two or more supernatural species. They started with a vampire being turned into a vampire, which was one of the types of hybrids. But within the TVD universe, there have been more.
They have some of the strengths and weaknesses of both their races. Klaus is the original hybrid because his mother had an affair with a werewolf, so therefore he had the gene and was later turned into a vampire. So Klaus is an original vampire as well as the original hybrid, which makes him the strongest villain in The Vampire Diaries and possibly the strongest creature as well, even though he is eventually defeated. His daughter Hope might be the most powerful creature to exist.