I got the overwhelming feeling when I had two horses (I has grow to two sims at this stage as well) that I wanted to put the one I had got from the adoption services back up for adoption. On the day I decided to jump the ring of fire with my other horse and caught fire. The horse threw me and I was quite a way from the house, and the fire service was useless. Anyway I died
And the adopted horse saved me from good old' Grim whilst the other one sat eating grass RIGHT NEXT TO ME!
Needless to say the adopted one stayed and I just tried to cope (I did, eventually) I was never as enamoured with the other horse after that, to be honest.