The Idea of a character transforming into a animal, monster, creature, demon, or other types of creatures.
Transformation or "Shapeshifiting" is a concept of a person transforming into an anime character, monster, animal, unknown creature or anything. It can also be as simple as changing your appearance. This is usually done by some form of magic, or science. Transformation has existed for many years mostly in fiction and stories. Usually someone evil transforms into a animal or monster to try and kill or fight someone. As the years went by Transformation has changed from just the typical villain shapeshifting to usually a hero.
In Japan there are many stories involving Transformation. One popular story is Kiyohime. Kiyohime is the story of a woman that fell in love with a priest named Anjin. But overtime he overcame his passions and refrained from further meetings. Kiyo became furious at the sudden change of heart, and pursued him in rage. The priest and Kiyohime met at the edge of the Hidaka river, where the priest asked a boatman to help him to cross the river, but told him not to let her cross with his boat. When Kiyo saw that Anjin was escaping her, she jumped into the river started to swim after him. While swimming in the torrent of the Hidaka river, she transformed into a large Dragon (in some stories a serpent) because of her rage. When Anjin saw her coming after him in the form of a huge Dragon, he ran into the temple called Doujouji. He asked the priests of Doujouji for help, and they hid him under the temple's bell. However, Kiyohime smelled him hiding inside the bell and started to coil around it. She banged the bell loudly several times with her tail, she then gave a great belch of fire that melted the bell, killing the priest.
Anyamaru Tantei Kiriminzuu - A new series where the main characters, the Mikogami sisters and their friends discover weird devices in their attic that give them the ability to turn into animals. Riko is able to turn into animals from the feline family, Rimu is able to turn into animals from the Leporidae (Rabbit) family, Nagisa is able to turn into animals from the Canine family, Ken is able to turn into a mouse and Tamao is able to turn into various flying animals. Later on more characters who are also able to turn into animals start appearing. Such as Kanon Hatori who is able to turn into a bat and her mother Misa who is able to turn into a panther.
Aquarian Age - The TCG series features a faction known as the Darklore who are woman that are able to turn into monsters from mythology. In the Aquarian Age: Sign For Evolution anime features a Darklore named Kanae Morino, a famous idol who turns into a Cat Darklore.
Berserk - In this series only a group of beings called Apostles are able to transform into a demonic form. Recently the lead character Guts was seen transforming into a Black Dog after using the Berserker Armor.
Devilman Lady - The series revolves around people turning into monsters or demons due too evolution process called the " Devil Beast Syndrome". The Main character is Jun Fudo who is a teacher in the manga while she is a famous fashion model in the anime. Jun is one of the few humans able to control her feral side and fights these humans who have gave into the feral side and devour and kill humans.
Fairy Tail - A series created by Hiro Mashima about Natsu Dragonil and his friends from the Fairy Tail Guild. Magic users are able to use two different transformation abilities. The first one is Take Over which allows the caster to take over the power of a creature or person and use it to fight. The second ability is simply Transformation which allows the caster to change their appearance by turning into a animal or other person. The only characters who mostly use these abilities are Take Over Siblings; Mirajane, Elfman and Lisanna. Another member of the Fairy Tail guild named Macao Conbolt is able to use the above abilities.
Fruits Basket - The Fruits Basket series revolves around the story of Tohru Honda, and the Sohma Family. The Sohma Family are cursed to turn into animals of the Chinese Zodiac when they are hugged by the opposite gender.
Himitsu no Akko-Chan - The story of a young elementary school girl named Atsuko Kagami (Akko) who one day breaks a mirror given to her as a gift from her parents. Instead of throwing it away she burries it in her back yard. Later that night she is visited by the Queen of the Mirror Kingdom, who is touched that Akko would treat the mirror so respectfully and not simply throw it away. Akko-chan is then given the gift of a magical mirror and taught an enchantment that will allow her to transform into anything she wishes.
La Blue Girl - A hentai series about sex ninjas. The series features a sex ninja named Yaku who is a werewolf that transforms during a full moon to prevent this she must reach an orgasm. Another Sex Ninja named Fubuki Kai is able to turn into a Shikima Demon after obtaining the Blue Blood of the Shikima.
Lives - A manga series by Masayuki Taguchi about a strange meteor shower that hits various places in Japan transporting people to a prehistoric world. In the strange world humans discover they are able to turn into monsters.
Marvelous Melmo - A classic series created by Osamu Tezuka, about young girl who mother died leaving Melmo and her brothers to fend for themselves, was given magical candy from God that could change her age or turn herself into a animal. Melmos brother Totoo accidentally eats both versions of the candy and is turned into a frog. Throughout the anime Totoo is stuck as a frog because his body is unable to consume the candy. Eventually his sister is able to find a way to return him to normal towards the end of the anime.
Outlaw Star - In the Outlaw series, there are a race of aliens called Ctarl Ctarl. They have the ability to turn into a tiger when there is a full moon (similar to a werewolf). One of the crew members of the Outlaw Star named Aisha is a Ctarl Ctarl. There is also a member of the Kei Pirates group the Anten Seven named Lady Iraga who is a werewolf.
Ribbon no Kishi (Princess Knight) - Another series created by Osamu Tezuka about a young girl named Princess Sapphire who was born with two souls one of a girl and a boy. Sapphire must pretend to be a male prince so she can inherit the throne. The series features two characters that are seen turning into animals. Satan who is a powerful Warlock that desires to obtain her two-hearted soul and take over the Kingdom transforms into vicious animals to fight Sapphire. Satan also has a daughter named Heckett who is a witch that helps Sapphire escape her father. To do this Heckett turns into a animal to secretly help Sapphire without her father knowing half of the time.