Don Hatt

Don Hatt is the former King of The Island of Sodor, although he was not born into the true royal family. He was the husband of Queen Audrey and maintained the title of King through his marriage with her. He was not born human, but actually as a frog, and became a human due to the Fairy Gothelmother's magic, however, when she died, so did her spells.

Early Life
Don was born a frog who lived in a pond. One day, he met up with the Fairy Gothelmother, who allowed him to pursue his dream of marrying and loving Princess Audrey, by making him human. Their first date was in the night and their first kiss made him human. After the death of Audrey's father, he became the king and married Audrey.

After the marriage, Sir Topham Hatt was born, but he had a illness. Gideon was close into getting King Don to sign a Magical Contract in order to supposedly remove Sir Topham Hatt's illness, however, just as the king and queen were about to sign it, he was told that Sir Topham Hatt was cured.

Thomas and Tillie
At the beginning of the film, Don doesn't like Thomas at all and is against the idea of Sir Topham Hatt adopting him, so he starts acting very antagonistic towards Thomas. He and Thomas argue at the dining table during a feast to celebrate the return of Sir Topham Hatt thus, destroying the feast.

He hears Thomas and Sir Topham Hatt having an argument, and blames Thomas for everything, even though he was being cruel to Thomas. Then, he was taken in the Fairy Gothelmother's carriage where she told him that he must have Thomas killed so her son Prince Charming can be adopted by Sir Topham Hatt and become the king.

Trivia

 * Don is possibly inspired by Alex Hirshe's grandfather.
 * Despite having barely any screen time in the last two movies, the credits still treat him as part of the main cast.