Saves from the October Next Fest demo of Horses of Hoofprint Bay carry into the full game. February demo saves are incompatible with the release, though you can still play them in the separate February demo branch. Identify the version that created your save before you switch.
Continue a save from the October demo
The October demo was updated to match the newer game systems. If that's where you started your yard, open the full game and look for that save before you create a fresh one. Check the horse and yard you expect before playing on.
Keep the February demo available
If you want to revisit a February save, open the demo's Properties in your Steam library. Under Game Versions and Betas, select the February Demo branch. Use the demo for that yard and start a new save in the full game.
Check the version when a save is missing
The 25-day limit applies to the October demo too, so the cap alone won't identify your version. Check which branch you played and whether you had October additions such as Caprilli. The developer's October demo announcement explains the compatibility split and the older branch.
FAQ
Can I carry over a February save by switching branches?
The February branch lets you keep playing that version. The developer only confirmed full-game compatibility for October saves.
Does a transferred yard have to stop at day 25?
The October demo has a 25-day cap. The full game lets you continue past it.
Do I need to finish all 25 days for my save to be compatible?
The developer describes compatibility by demo version and gives no day-completion requirement.