Maybe not really new to you who played Pets a lot...
I finally befriended an unicorn, mounted him and ride him home (silly me, I could teleport as the sim was complaining about his energy being low).
Next day, I decided to give him warm velcome, and feed him something tasty. Unfortunately, the sim I sent to him had only one item useable for this - Life fruit. I go like, whatever, warm welcome, I can grow more of them anyway, and fed it to him...and was really surprsed that I got message like "Indigo has eaten a Life fruit and will live longer" and his age bar moved 1 day back.
So if you're gardener, with good supply of Life fruit you can make your horse immortal.