I am, also, having some fun with the Legend Editor.
The file one creates is a 'lx.db' file which is saved in:
Documents/StartersOrders7/Legends (win 10)
I notice when creating new Legends one has to set their age to 1 if one wants that particular horse to appear as age 2 at the start of the game.
Not sure why that is (bug?), but at least it works.
Of course, you have to start a new game for the edits to take affect
& REMEMBER to set those new Legends to 'inc' (far left) so as to include them in your new game.