Transgender MMA Fighter Fallon Fox -- Picture of Pre-Op Manhood

Transgender MMA fighter Fallon Fox was a stereotypical manly man before she became a woman ... and TMZ has obtained the photos to prove it.

An ex-GF tells us ... she met Fallon back in 1999 when he was serving in the Navy. Fallon -- born Boyd Burton -- went by "Woody," after "Woody Boyd" from "Cheers."

The ex says Woody was a "manly man" -- athletic, short-cropped hair, leather jackets, big into heavy metal -- and she never suspected he'd become a transsexual. The two eventually broke up because of distance ... no other relationship problems.

Years after the breakup, the ex says she reached out to Fallon after she announced her transition on Facebook -- and that's when Fallon informed her she always felt she was in the wrong body.