That you become the mare yourself. Which is exactly what a Norwegian fan did just recently when he decided to go through a process of changing his name.
Recently, a local pony fan from Norway decided it was a good idea to change his legal last name to "Twilight Sparkle."
According to a Norwegian Facebook group, Eimar Twilight Sparkle decided he wanted to have a unique name and went through the process of doing so. According to Norwegian law, he cannot change it back for at least ten years too, so for better or worse, he's stuck signing legal documents as Twilight Sparkle.
We can only speculate how this is going to affect ships, and probably how Jin is going to react, seeing as he believes he's married to Twilight Sparkle. There are so many questions!