a tranhorse is when a persons neck and head are forward of their body. it is like a hunch back however the back isn't involved in the forward propulsion of the head - just the neck.
Tran - derived from men who become women stoop with their necks as they have tried to hide themselves whilst uncomfortable with their gender.
Horse - because it is rather like a horses neck the way it bends forward.
"that boy has a hunch back in his neck"
"no - that's called a tranhorse"