Bless Unleashed

You get a mount early on in the story, and you can get one before you even create a character by linking your gamertag to a Bandai Namco account, (you can make one in the game client). You can also find mount merchants in the big towns and buy a mount from them once you have enough gold. As for actually getting the achievement, I got it in the second big region. That is to say this is not hard at all and you should get it way before you get close to finishing this game, so don't sweat it.