Even though chronologically it was already their third commercial release—preceded by two mini-albums (The Day Dragged On and Public Garden) earlier in the year—nevertheless, due to its full-length nature Mustang! is considered as the group's debut album, also marking their first major commercial release into the Japanese music market.
Although they had released two mini-albums ("The Day Dragged On" and "Public Garden") earlier in the year, "Mustang!" is recognized as the band's debut album because of its full length format and its significance as their first major commercial release into the Japanese music market.