Nintendo of America's release trailer for the game includes a couple of horses:

it's a little unituitive, so here're some instructions

1. Make sure you have both the demo and the full game installed.
2. Then you need to kinda initialise the full version by just starting it up. Press the 🏠 button and then close the game.
3. Open the demo and you should be asked if you want to transfer your save.
4. Once done you can delete the demo and its save data :)

• https://wiki.dengekionline.com/solitiba/ - Japanese wiki.