There are 55 actual legends in the Legends set. That number is divisible by five, so there must be some sort of cycles present, but with all the different colors in their mana costs, it's hard to sort them out.
But there is indeed a pattern. Each allied three-color combination is headed up by one (rare) Elder Dragon Legend, and beneath him are three other rare tri-colored legends. Beneath them, the colors split into allied pairs, and each pair is represented by three more rare and four uncommon legends.
In the example below, the "family tree" is diagrammed for the blue/white/green legends, headed up by "Mr. Castle," Arcades Sabboth. Each three-color combination can be laid out just so. If you're ambitious, try building decks that utilize all the members of each family!