The Showa series (昭和シリーズ?, Shōwa shirīzu), also known as Showa era and Showa period, is a term used to identify the years between 1926 and 1989 under the reign of Japanese Emperor Hirohito.
The first Godzilla film was to begin the Showa era of the kaiju industry, and is the usual kaiju to be affiliated with this era. During the beginning of this era, Godzilla was mainly antagonistic in nature, taking on the likes of Anguirus and Mothra. By the time of Ghidorah, the Three-Headed Monster, Godzilla adopted what would soon become his most famous image: a heroic monster, battling terrible villains with other monsters by his side.
This is the longest of the current four generations, lasting over 40 years.
Politically, The Return of Godzilla is a Showa film, as it was the last film to be made in the Showa era of Japan, with the Heisei era beginning in on the 8th of January 1989.
This era features the majority of appearances for several monsters. Rodan, for example, appears in four films (or around 10, including stock footage appearances) in the Showa era, whereas in the Heisei and Millennium eras, he only has one appearance in each.