6. Momaw Nadon (Hammerhead)
What you see in the films: Back to the cantina in Episode IV we go, as everyone remembers this large and unique looking alien. His action figure was released in 1978 by Kenner, and is the first place we saw him referred to as Hammerhead (his production nickname, which most background characters had).
The expanded backstory: Hammerhead is a member of the docile, environmentally conscious Ithorian race, and is actually named Momaw Nadon. His people live in floating cities (kind of like Bespin) to avoid damaging the plant-rich surface of their home planet. Momaw Nadon had revealed his race's advanced botanical knowledge to the Empire in an attempt to save his planet from attack. The Ithorians thanked him for saving them by banishing him forever, which is how he ended up on Tatooine.