No one knows for sure. But there are at least two conflicting theories.
The most popular is that it was named after a medieval Portuguese sailing vessel, called a caravel, since the marine colonial hydrozoan also plies the open oceans using its pneumatophore (float) as a sail. However, since the term 'Man of War' specifically refers to a later British design, you would have to assume that the seventeenth century term for an English fighting ship was applied to the fifteenth century Portuguese scouting and trading vessel. And, you would also be hard pressed to see any similarity between the creature's rounded sail and either a lateen-rigged (triangular shaped) caravel sail, or a square-rigged carrack sail.
What seems much more plausible to me is that it is named after the helmet worn by Portuguese soldiers (men of war). That helmet is called a comb morion. If you Google an image of the helmet, you will see a striking similarity in appearance. Just saying, but then I also think fluorescent road cones look like psychedelic witches emerging from the center of the earth.
the Korean war is also called the forgotten war.
The War between Bismarck and the Danes was called the Second Schleswig War.
it waz called the hundred years war
They were called conscientious objectors.
it was called the war of independence because the colonies was fighting for freedom from the British rule
According to http://www.merriam-webster.com/dictionary/manofwar "Men-of-war" is the plural.
Manof was created in 1980.
World War I was originally called "The Great War" (not the Civil War)
the Korean war is also called the forgotten war.
It was called the Civil War. I think you are confused.
The War Between the States was also called the Civil War.
It was also called the Great War.
The war in 1914, originally called the Great War, was later called a world war as it was fought by more countries and on a larger scale than any war before.
Pro-war members of Congress are called hawks; anti-war members are called doves.
The American Civil War, sometimes called The War between the States.
Both the American Civil War and World War One have been called the First Modern War.
World War I It was also called "the war to end all wars". Guess they were wrong about that.