Tin Men in the Books
Both were originally humans; However, they were enchanted by the Wicked Witch of the East's magic to chop off their own body parts. Each time this happened, they hired a tinsmith to make them replacement parts out of tin. This continued until they were both entirely made of tin, with no flesh left.
Tin Man Tin Men
These Tin Men are actually normal humans, who protect the Mystic Man; they are a police force of sorts. Wyatt Cain was a tin man, before joining a resistance against Azkadellia. He was imprisoned and forced to watch his family's torture over and over.