When did the Phoenix first appear? It is probably even more confusing than the research on dragons.
The Classic of Mountains and Seas states: "There is a bird, its shape like a chicken, with five colors and patterns, named Phoenix Sovereign."
From this sentence, the Phoenix Sovereign appears to be a type of bird. However, also in the Classic of Mountains and Seas, it is recorded: "There are three birds with five colors, one called Imperial Bird, one called Luan, and one called Phoenix."
This then turns into three different birds, and there is still no consensus on which is the correct one.
As for later generations, people usually interpret the phoenix and the sovereign as the male and female of the same species, which is actually a manifestation of monarchic symbolism.