Depends on your horses stride! Decide the pace you want for your freestyle and how you want your energy for your ride to be which really depends on your horses ability to have their pace changed, just find a song that's beat matched your horses stride!
What about the a mashup of Fishin in the dark and boondocks sung by Home Free
I think a fast paced country song works well with freestyle. I have a family friend who did a routine with the song "any man of mine" by Shania Twain lol