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.
"Mustang!" is considered the band's debut album despite being their third commercial release because of its length and its significance as a major launch into the Japanese music market.