A mullet is a type of fish, and I hope that our esteemed biological categorists will give a better description, including a link to the appropriate genus/family/species.
A mullet is also a type of haircut, long associated with
United States redneck culture. Short on top, long (shoulder length or more) in back, very often combined with a moustache. The term was supposedly first coined by the
Beastie Boys.
A mullet is slang for a person that mindlessly follows a fad/trend/leader.