Viking ships had dragon heads because dragons were considered to be powerful and feared creatures. Vikings believed that having a dragon head on their ship would make it more powerful and feared by their enemies.
The dragon heads on Viking ships were used as a symbol to strike fear into the hearts of their enemies. The dragon was also a symbol of power and strength, which would have made the Viking ship seem even more imposing. The heads might also have been used to help guide the ship through rough waters.