Huntail

Huntail is a unique aquatic Pokemon from the Hoenn region. It is a dual Water/Dark type Pokemon. It is an evolved form of a Clamperl. This can be done by trading Clamperl away while it's holding a Deep Sea Tooth.

Its alternative is Gorebyss, a pure Water type. It can be done by trading away Clamperl while it's holding a Deep Sea Scale.