You may have already understood that I prefer to take one character and put it in different AUs and make different designs for them. It depends on the nature and events in the character's life in a particular AU, and made a note with explanations of the conventions of the world: https://www.furaffinity.net/view/38407065/
This Markus (in this AU his name is Mark) was born as a snake in a bird family and his father hated him. Markus's father, Joshua, was a religious fanatic and a cruel despot who never doubted the correctness of his decisions. And when Markus was 17 years old, Joshua went completely insane and tried to kill his son. Markus fought back and accidentally killed him. Unfortunately, too many people turned a blind eye to Joshua's crimes, and therefore it was more profitable for them to jail Markus, who at the time of the investigation was already 18.
The killing of his father and imprisonment traumatized the boy and he made a traumatized transition that led to the combination of the appearance of a reptile and the appearance of a bird.