Andromedan Sets are pairs or triplets of dragons that are considered one entity, bound together by magic. They are best understood as single dragons, but with multiple bodies. Though the first dragon that existed as an Andromedan Set to be discovered was the Sprout Dragon, Andromedan Sets are named for the Andromedan Dragon because "Andromedan set" was a better name than "Sprout set," which, according to The Great Nogard, sounds like one of those trays of seedlings you can buy in the Market.
Interestingly, the three known dragons that exist as Andromedan Sets each function very differently in terms of the interaction between the members of the set. In the Sprout Dragon, the dragon begins with a single body, which later multiplies as the dragon grows. The members of the set themselves do not age or change over time; they essentially remain babies forever. The andromedan dragon has two bodies when it hatches, both of which grow and develop like any other dragon, and both of which can perform all the functions a dragon needs to live independently. The Fates Dragon, like the andromedan dragon, possesses more than one body when it hatches, but unlike the andromedan dragon, each body needs the others to survive, because each body cannot on its own perform all the functions a dragon needs to survive.