I know and realize many players either don't or can't have mods, but I solved my problem ages ago. I first had my Sims' two brothers who bought a house together keep Erik's two horses for him. His household was already maxed about before Horse Ranch was even announced, let alone released. Which kid was I to part with?
At any rate, having Joey and Aaron care for them was a temporary solution until I decided to have Erik move his family to Chestnut Ridge. This came out of a slight plot twist the game handed me. So, I moved Erik to the new place, and he brought his horses with him. I used MCCC to increase the size of the household, then I moved the rest of his family to the new place.
Yes, it's a pain in the assets to be unable to take them into cas.fulleditmode at will, without losing the number of Sims/horses over 8, but I learned a work-around for that, too. I just sent him to his brothers' house, had them ask him to join their family, take him into cas,fulleditmode, and in this case remove the unsightly muscle bulk from his relentless workouts.