This ought to belong in the collection of Pokémon Lecture, but since it is Halloween, it deserves to be its own separate story. Anyways, I don't own Pokémon.
In the Pokémon world, ghosts are not a mere fantasy or something that can be disproven, they are a blatant fact. Ghost Pokémon are the fewest of all Pokémon types, having only 27 known species. However, they are one of the trickier Pokémon to battle. Many trainers and Gym Leaders (most notably Agatha of Kanto Elite Four, Morty of Johto Ecruteak Gym, Phoebe of Hoenn Elite Four, Fantina of Sinnoh Hearthome City Gym, and Shauntal of Unova Elite Four) have specialized in this rarer type. Ghost Pokémon may be found in cemetery towers and sometimes simply after the sun goes down. It is said they are never far away from naughty children nor from angry, bitter people.
Every ghost Pokémon can become transparent and slip through walls as a spirit does. Some even specialize in hiding in invisibility and evading nearly all opposing attacks with their skill. Most can even possess living and non-living things. They all are immune to normal and fighting attacks and can possibly evade any other kind of attack without a foresight attack to ensure a hit on a ghost.
But there are more mysteries surrounding this type of Pokémon than any others. Many a researcher has asked, "What is a Ghost Pokémon? Are they human? Are they spirits of dead Pokémon? If so, how are they able to breed and create offspring? If they are dead Pokémon, is it possible for them to die again or go to the spirit world?" Not much of science can clarify this oddity. Mere speculation and professor's research has brought us this far.
The most common and oldest-know ghost Pokémon is Gastly and its evolutions. It seems quite odd that such different Pokémon can evolve into each other. Gastly, a mere ball of gas that it thought to be similar to a Shelder evolves into a Haunter with floating arms and a killing lick, then finally becomes a solid Gengar that lives in shadows and drains away life. It is strange to think a spirit of a Pokémon or human could be reborn as a frail gas Pokémon, but it is said that most ghosts that appear to us in the Pokémon world are either real ghosts or Gastly in disguise. But what is a real ghost in the first place? That will be a different topic.
Misdreavus was the first discovered pure ghost type Pokémon. It resembles a head floating around without its body, and it has a nasty habit of screeching unearthly wails and pulling on people's hair. It has been proven that it feeds off of fear and can eat it like Pokémon food. This trait is shared with Shuppet. In evolution, Mismagius gains the ability of incantations, which humans have tried to copy in the past. The design of Mismagius and their incantations became transferred on people who professed to do magic without a Pokémon's aid and were called witches.
There is a strong misunderstanding about the origins of Shuppet, because their evolutionary superior form is said to have come about from a neglected toy. That brought up a certain amount of confusion regarding its birth. It is still known Shuppets regularly feed off of negative emotions like a snack and it can be guessed that Shuppets simply feed on negative energy so that in their older form they release it. Banettes are vengeful spirits that have hints of voodoo magic (a needle-like tail sticking in it), a zippered mouth that supposedly keeps all of their malice inside.
The Duskull family demonstrates a leaning towards defensive abilities rather than special attacks. They are most thought of as departed souls from their skull-like heads, single eye, and arms in their later stages. It is a popular belief that Dusclops is in fact a real mummy (one that could be found within a Cofagrigus). However, as with most ghost-type studies, there are yet to be solid answers on how close a Duskull is to a human's soul.
In the Unovian region, a species of ghost-type was found to be truly a human's soul: a Yamask. It is speculated that it is unethical to train such a Pokémon that was once human, as is the issue for most ghost Pokémon trainers. Even worse in the case for training a Yamask is the fact that Professor Juniper found that a Yamask remembers its time as a human. There will be no doubt a legal battle coming up on the rights of Yamask.
Many other ghost Pokémon like Drifloon, Litwick, Spiritomb, and Sableye have connections with eating emotions and kidnapping children at certain points. (As much as I would love to go into the facts of Spiritomb and Sableye's complete resistance of all weaknesses, that will be for another time.) Shedinja seems to be one of the purer Ghost Pokémon to study simply because it is regarded as a living shell of a Nincada and it therefore a ghost of a Nincada that was not contained by a Ninjask. Rotom also offers insight in the shifting shape and abilities of a ghost, including appliance possession.
Still, after all of the differences of these ghosts, there hasn't been any clear evidence on how these Pokémon came about (with the exclusion of Banette and Yamask) and how they are able to act like normal Pokémon in a daycare. Baby Gastlies are not unheard of, which brings up the problem of whether a Gastly is truly a dead soul. It is feared that if a Pokémon Breeder took up specializing in the Yamask species, people will start dying and being born as Yamasks. Regulations on the Unovian ghost are being worked on currently.
Not even the prestigious Gym Leaders and Elite Four members fully understand their own Pokémon's past and origins, despite all of their cooperation with these mystical Pokémon. It is still up to budding researchers and trainers to discover more and more about the Pokémon races and their roles in life. Some say that ghost Pokémon are a way of keeping a little fear in the world and are able to take away angry emotions. Others say Ghost Pokémon are a sure sign of an afterlife and that a soul is separate from the body. Neither of these has been -nor can be- proven, but they are still fascinating concepts.
However, despite folklore and tall tales, Ghost Pokémon are still classified as Pokémon and they evolve and grow like other regular Pokémon, so there can be a slight peace about that fact. Whether they are Pokémon souls or a completely different kind of Pokémon altogether, they still are one of the Pokémon types and are able to be trained.
Thank you for reading. Happy Halloween! And a thank you to birdboy for his writings of Gastly, Haunter, and Gengar.