I agree that you need to know the tempo of your horse's trot. I think there are websites that can suggest music for various tempos. Try using creative Googling.
I have always thought that the opening sequence of Sgt. Pepper's Lonely Hearts Club band song would make a really cool entrance to a musical freestyle.
Or, maybe a really cool Pas de Duex.
I know it's probably all the wrong speed.
Man, it IS not a good speed. Oh well, it's a fun song anyway. Enjoy!