Note: While this illustration is probably still under copyright, I believe that this display falls under fair use, and is not a copyright violation. Of course, given the current state of copyright law in America, it's really anybody's guess.
Second note: In case it is not obvious, there is no connection between this website and Del Rey, the publisher of the book pictured above. The Del Rey people seem pretty cool, and they publish a number of great books, but my only connection with them is that I buy a book from time to time.
Panthan
The term panthan comes from the Martian Tales of Edgar Rice Burroughs (slightly better known, perhaps, as the creator of Tarzan). In 1911, the first book written by Burroughs appeared. It was A Princess of Mars, the first of the eleven books that chronicle the adventures of John Carter, who becomes (in the third book), the Warlord of Mars.
(To be truthful, several of the books mention Carter only briefly, if at all.)
Panthans on Mars are wandering warriors. These are men who for any of several reasons have had to leave their home cities, and therefore survive by hiring their swords. There is none of the negative connotation of the modern term "mercenary"; it is understood that panthans are in general honorable soldiers who have left home for many reasons, few of them having to do with shame.
Links are to the texts of the books at Project Gutenberg, one of the best uses of the web. (Due to the most recent Copyright Extension Act, passed by Congress for the sole purpose of protecting the Disney corporation from loss of profits, the last six books are still not in the public domain. The first five are, however.)