Headcanon: Wingly Killian
Killian was born in a time when Winglies (creatures with platinum silver hair who can fly using wings that appear at will. Have the ability to use magic) ruled over the world of Endiness. However ever since he was young he felt like it was wrong to deem themselves superior to humans. He was always taught that humans were lesser than them. His mother died not long after he was born and his father began drinking regularly before abandoning him and his brother.
He and his brother joined the Wingly Army when they were old enough, but Killian often found the missions they were given to be cruel. Often involving putting humans in their place. In an attempt to sway Killian to believe humans were awful, they were sent on a suicide mission that ended with his brother being killed. However Killian always blamed the commander for sending them when he knew that they may die.
Then Killian fell in love with a human woman. This had been frowned upon by the other winglies and they ended up killing his love. However upon finding her dead and knowing that the other winglies killed her, he felt his home in search of a new life.
He then went to another world, (Enchanted Forest), where he became a pirate. However due to the unnatural silver color of his hair, many seemed to deem him strange. So he opted to start using henna and occasionally khol to cover his silver hair color. From then on he blended in with the humans of this new world, never letting anyone know of his true nature.
In this new world he fell in love again, this time with a married woman (Milah). She left her husband for him and she had been the only person who ever knew that he wasn’t human. However her husband found him years later. . . a changed man and killed Milah, cutting off Killians hand. He attempted to kill the monster using a fallen hook. However the man was not harmed. In a desperate attempt to kill the man who took his love he used his magic then to attack him, shocking both the Dark One and his crew. However it was not enough and he left Killian to mourn the death of his love.
From then on he became the pirate Captain known as Captain Hook. And though his crew had been surprised by his use of magic, they did not question him or how he had been able to do that. Nor did he offer up that he was not human.