I have never thought about it in the terms Pete describes i.e. "Transgender" but I agree 100% with Pet's analogy
What I do know is that every champion mare I have ever seen is very masculine, this makes them very good racehorses but very bad mothers.........Going back to Emancipation, Let's Elope, Makybe Diva, Sunline, Black Caviar to Winx they were all masculine, that is why Champion mares never throw foals that can gallop, as for being "transgender" I am not sure but I can see the logic behind the metaphor
Comparing Humans to racehorses is ignoring the facts, Humans are mongrels in a breeding sense, there is no planning in breeding humans, Racehorses on the other hand are Thoroughbreds, they all pretty much trace back to the same spot in their history, the Darley Arabian, the Godolphin Arabian and the Byerly Turk. ...there is much less variance in the Thoroughbred than there is in Humans, Thoroughbred is not just a word plucked from the ether, it is an accurate description of a Racehorses pedigree, you cannot mate a thoroughbred mare with any other breed and register it as a Racehorse/Thoroughbred, it wouldn't be thorough!