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A series of animated short films in the long-running Ultra Series, consisting of two films, Ultra Nyan: The Mysterious Cat that Fell from the Starry Sky, and Ultra Nyan 2: The Great Happy Operation, which were both released in 1997 and 1998, respectively.
Both shorts are paired with two different Ultraman movies, as in the first short being shown before Ultraman Zearth 2, whilst the second short being shown before Ultraman Tiga & Ultraman Dyna: Hikari no Hoshi no Senshi-tachi in movie theaters.
This tag implicates ultra_series (learn more).