John "Eleanor" Rykener... Uh...
John Rykener could not possibly be the first transvestite but he is the first documented one, and he is a prostitute. He calls himself "Eleanor." He has been arrested for committing lewd acts in a horse stall and has confessed to accepting money from women and men for sex including members of the clergy because they pay more. Cross-dressers are not always bisexual but in this case, it was how "Eleanor" made his living.  
This Year on Wikipedia
Year 1395, Wikipedia.
- Halsall, Paul. (translator) The Questioning of John Rykener, A Male Cross-Dressing Prostitute, 1395 - Internet History Sourcebooks Project. 1998-May. (TRANSCRIPT) quote="Rykener further confessed that many priests had committed that vice with him as with a woman, how many [he] did not know, and said that [he] accommodated priests more readily than other people because they wished to give [him] more than others."
- John Rykener - Wikipedia, 2014 [last update]
- Edward II of England: Piers Gaveston and sexuality - Wikipedia, 2014 [last update]