To me the horse of a lifetime (so far), is one I only knew a few short months and was never mine. I used to ride at a ranch, and I would exercise others horses for them when they themselves couldn't.
One day, there was this horse named only "Black", owned by someone else, but I rode him for them. Man, was he something else. FAST, but still controlled(ish), good go all day and LOVED to just lope circles. It was like riding a sports car, or the sky, or something I don't even know. He also had issues, he had marks from where a halter had grown into his face, and he was so headshy it took many, many tries to get his bridle on. But I adored him, I worked with him, teaching him confidence, how to control himself under saddle, and we bonded. He was impossible to catch in a paddock, but I could catch him.
I loved him like my own (actually, even more than my own...)
Then, one day, his owners took him off the ranch, I had an evening to say goodbye, so we rushed to the ranch, and I rode him, hugged him, and for the first time, right before I said goodbye, he put his head in my chest and just stayed there.
He was gone a long time, I thought I would never see him again and I really mourned, not knowing if he was being abused like his past, or what or where he was.
One day out of the blue, we got a call from the ranch owner; "blacks back". I cried I was so happy, I couldnt wait to see him, ride him, see if he remembered me. When I first saw him again, he did remember me and I was overjoyed. His owners were selling him, and my family and I were figuring out how to buy him.
As we were, the ranch owners daughter decided she wanted him, though she had never ridden or interacted with him like I had. Everyone knew how much I loved him, it was a well known fact I adored and would buy him. She bought him however, renamed him, and basically, ran him into the ground. He became high headed, crazy under saddle, and not the horse I once knew. I barely even recognized him. We left the ranch a few years later, but I still miss him, and wonder what we could have become if I could have had him.