It is a simple Pirate's cutlass and a magically-infused flintlock pistol. Befitting Tetra's origins as a seafaring pirate captain, the Pirate Cutlass has a Water Elemental Attribute and can fire bullets that can create a pool of water that break down enemy defenses.
Theory warning: This section contains theoretical information based on the research of one or several other users. It has not been officially verified by Nintendo and its factual accuracy is disputed.
Its presence in Hyrule Warriors & Hyrule Warriors Legends as a weapon used by the pirate Tetra is likely a reference to use of both flintlock pistols and cutlasses by pirates during the Golden Age of Piracy.