Mamoru is Setsuna
Mamoru is Setsuna refers to an internet legend coming from the early days of the newsgroup alt.fan.sailor-moon. No one is quite sure who originally spawned the meme, but it took hold of the minds and hearts of the fans and has refused to die.
The perceived origin of the legend is based on the false premise that Setsuna Meiou and Mamoru Chiba never appeared on the screen at the same time in the anime, and that they were drawn strangely similarly. The length of Setsuna's hair and the darkness of her skin were explained by use of cosmetics.
As this legend grew, this caused the growth of other, related legends to appear, the most controversial of which was that Chibiusa was Mamoru and Setsuna's daughter, not that of Mamoru and Usagi at all. The exact details of how this occured were left to the imaginations of the readers of the newsgroup, and discussed at length by the members of the group in a tongue-in-cheek manner.