Transgender is a blanket term to describe all gender-variant persons. It formerly only referred to someone living as the opposite gender without the benefit of surgery. Transgender variations are likely to appear in any society adhering to the following exclusive binary logic:

man + woman ==> marriage ==> child
male child ==> boy ==> man ==> marries woman
female child ==> girl ==> woman ==> marries man

Transgender variations lead to alternate versions of current genders and even new genders. In societies where gender is not binary, there may not be any transgender variation.