Kaijū (怪獣 kaijū?) (from Japanese "strange beast") is a Japanese film genre that features monsters, usually attacking major cities and engaging the military and other monsters in battle. It is a subgenre of tokusatsu (special effects-based) entertainment. This word originated from Shan Hai Jing (The Classic of Mountains and Seas).
There are no traditional depictions of kaiju or kaiju-like creatures in Japanese folklore but rather the origins of kaiju are found in film. Gojira (translated to Godzilla) is regarded as the first kaiju film and was released in 1954. Tomoyuki Tanaka, a producer for Toho Studios in Tokyo, needed a film to release after his previous project was halted and upon seeing how well American Hollywood monster movie genre films King Kong and The Beast from 20,000 Fathoms had done in the box offices of Japan, as well as personally being a fan of the films, Tomoyuki Tanaka set out to make a new movie based on those American monster movies and created Godzilla. This meant the original Godzilla movie, and the entire kaiju genre is heavily influenced by Hollywood monster movies. Tomoyuki Tanaka aimed to combine Hollywood monster movies with the re-emerged Japanese fears of atomic weapons, which came about due to the Daigo Fukuryū Maru fishing boat incident, and so he put a team together and created the concept of a radioactive giant creature emerging from the depths of the ocean which would become the iconic monster Godzilla. Godzilla was initially met with commercial success in Japan, inspiring an entire genre that came to be known as kaiju movies.