Hey Lucara, glad to hear you found Samson. Do they remember? Well they certainly seem to when someone has been cruel so why not kind?
I started a morgan filly years ago for the owner and after I left it had been a year or so since I had seen her. Went back to pick up some books and few other things. She was out in the pasture with the other horses and the owner wanted to show her to me. Wouldn't come to the owner so she went into the barn to get a lead rope.
Well, I stood there and noticed she (the filly) was looking at me so I gave her the cue to come I used on her when I worked with her. By gosh, she came right up to me and put her head on my chest as if to say, "where have you been!" So I guess I believe they do remember you.
Good luck with him, and remember he will also reflect the handling he has experienced while away from you. I'm sure he'll be fine, but be alert and careful.
AZ was raised by my Mom for his first 2 years. She spent a lot of time walking him around, giving him treats, and cuddling him. He is a pretty friendly horse, but he won't "cuddle" unless he knows you (he cuddles by putting his whole head in your arms).
She came to visit after not having seen him in about 3 years, and he immediately cuddled her. This was after she used the distinct whistle she used to use to call him in from her pasture. He came right in even though we don't use that whistle.
I think he remembered her because she always treated him so well.
I think they remember stimulus and situations more than a person or another horse. Once you start riding and working a horse, I'm betting it will remember you more than just walking up and saying "howdy". As said by goldilockz , that horse remembered the cue. It didn't see the old owner from across the field and come running out of love and fond memories.
I have gone to visit my first born several times. My feelings are always a bit hurt when she doesn't seem to remember me. In reality, I'm glad she doesn't have that capacity. It would make the world worse for horses than it already is if they remembered being taken from their mothers at young ages and being hauled from owner to owner as so many are.
I believe they remember people in their lives. I rescued an AQHA mare when I first came to the US. She was about 300lbs underweight. I spent months working with her. Unfortunately I had severe complications with a pregnancy and I had to find her a new home. A couple of years later I heard she was up for sale again, though WAY out of my price range. But I went to see her anyway after explaining to the seller why. She remembered me, I'm sure of it. She nuzzled under my arm like she always used to when I was grooming her. I asked if she had done that at all with the new owner and she said no, she'd never seen it.
There was an article in Equus magazine a couple months ago about a horse memory experiment. They (some vet school, if I remember) trained a set of horses to recognize and pick out shapes and then re-tested them 10 years later and they still remembered the correct shapes to pick out.