I recommend saving the game before trying, but the following worked for me:
1. If you've lost one of the DLC horses you'll have to "lose" the other one too, I tried step 9 before I did that and it didn't work.
2. To "lose" the other DLC horse I rode with it out of the stables and then found me some wild horses, broke one and made it my primary horse by resaddling it. This should make the DLC horse your temporary horse.
3. Break another wild horse and bond with it until it's your temporary horse, that should make the DLC horse go to to theater or something - away from your stables, at least.
4. Save the game. Autosave will do, but to be on the safe side I recommend hard-saving and using a different slot than you're used to.
5. Close the game (the PS4 should do so for you anyway in step 7).
6. On your PS4 menu go to "Settings".
7. Go to "Account Management".
8. Select "Restore Licences".
9. Load your save and go to the Valentine stable, you should be able to stable them like you did when you first got them.
10. (OPTIONAL) Yell at R* for making us jump through hoops to get our stuff back. YES, I messed up by losing it first (my fault, though the game could have warned me) but it's digital - it should not be such a big deal to replace it in-game.
In my desperation I tried this, and it worked for me at least. Hope this helps.
PS: Yeah, I know you can probably just kill the DLC horse in steps 2-3, but I figured I'd just do again what made me lose the other one in the first place...
